Added release of byte buffer in PacketEncoder.java

Hi,

I was running the new 1.7.2 version in our performance tests env and keep getting a netty resource leak (See stack trace). I added a fix for the leak and thought you might wan't to take a look at it. 

With 1.7.2 I get the error almost instantly when we have ~100 clients, but with the fix we can't reproduce the leak.

Please have a look at it.
Thanks.

2014-08-28 12:50:56,631 [ERROR] (pGroup-3-2) io.netty.util.ResourceLeakDetector - LEAK: ByteBuf.release() was not called before it's garbage-collected.
Recent access records: 2
#2:
	io.netty.buffer.AdvancedLeakAwareByteBuf.toString(AdvancedLeakAwareByteBuf.java:697)
	com.corundumstudio.socketio.protocol.PacketEncoder.encodePacket(PacketEncoder.java:271)
	com.corundumstudio.socketio.handler.EncoderHandler.handleWebsocket(EncoderHandler.java:212)
	com.corundumstudio.socketio.handler.EncoderHandler.write(EncoderHandler.java:191)
	io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:658)
	io.netty.channel.AbstractChannelHandlerContext.access$2000(AbstractChannelHandlerContext.java:32)
	io.netty.channel.AbstractChannelHandlerContext$AbstractWriteTask.write(AbstractChannelHandlerContext.java:939)
	io.netty.channel.AbstractChannelHandlerContext$WriteAndFlushTask.write(AbstractChannelHandlerContext.java:991)
	io.netty.channel.AbstractChannelHandlerContext$AbstractWriteTask.run(AbstractChannelHandlerContext.java:924)
	io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:380)
	io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:357)
	io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:116)
	io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:137)
	java.lang.Thread.run(Thread.java:744)
#1:
	io.netty.buffer.AdvancedLeakAwareByteBuf.writeBytes(AdvancedLeakAwareByteBuf.java:571)
	io.netty.buffer.ByteBufOutputStream.write(ByteBufOutputStream.java:66)
	com.fasterxml.jackson.core.json.UTF8JsonGenerator._flushBuffer(UTF8JsonGenerator.java:1862)
	com.fasterxml.jackson.core.json.UTF8JsonGenerator.close(UTF8JsonGenerator.java:1087)
	com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:2720)
	com.fasterxml.jackson.databind.ObjectMapper.writeValue(ObjectMapper.java:2177)
	com.corundumstudio.socketio.protocol.JacksonJsonSupport.writeValue(JacksonJsonSupport.java:234)
	com.corundumstudio.socketio.JsonSupportWrapper.writeValue(JsonSupportWrapper.java:69)
	com.corundumstudio.socketio.protocol.PacketEncoder.encodePacket(PacketEncoder.java:268)
	com.corundumstudio.socketio.handler.EncoderHandler.handleWebsocket(EncoderHandler.java:212)
	com.corundumstudio.socketio.handler.EncoderHandler.write(EncoderHandler.java:191)
	io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:658)
	io.netty.channel.AbstractChannelHandlerContext.access$2000(AbstractChannelHandlerContext.java:32)
	io.netty.channel.AbstractChannelHandlerContext$AbstractWriteTask.write(AbstractChannelHandlerContext.java:939)
	io.netty.channel.AbstractChannelHandlerContext$WriteAndFlushTask.write(AbstractChannelHandlerContext.java:991)
	io.netty.channel.AbstractChannelHandlerContext$AbstractWriteTask.run(AbstractChannelHandlerContext.java:924)
	io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:380)
	io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:357)
	io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:116)
	io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:137)
	java.lang.Thread.run(Thread.java:744)
Created at:
	io.netty.buffer.UnpooledByteBufAllocator.newDirectBuffer(UnpooledByteBufAllocator.java:55)
	io.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:155)
	io.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:146)
	io.netty.buffer.AbstractByteBufAllocator.ioBuffer(AbstractByteBufAllocator.java:99)
	com.corundumstudio.socketio.protocol.PacketEncoder.allocateBuffer(PacketEncoder.java:51)
	com.corundumstudio.socketio.protocol.PacketEncoder.encodePacket(PacketEncoder.java:266)
	com.corundumstudio.socketio.handler.EncoderHandler.handleWebsocket(EncoderHandler.java:212)
	com.corundumstudio.socketio.handler.EncoderHandler.write(EncoderHandler.java:191)
	io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:658)
	io.netty.channel.AbstractChannelHandlerContext.access$2000(AbstractChannelHandlerContext.java:32)
	io.netty.channel.AbstractChannelHandlerContext$AbstractWriteTask.write(AbstractChannelHandlerContext.java:939)
	io.netty.channel.AbstractChannelHandlerContext$WriteAndFlushTask.write(AbstractChannelHandlerContext.java:991)
	io.netty.channel.AbstractChannelHandlerContext$AbstractWriteTask.run(AbstractChannelHandlerContext.java:924)
	io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:380)
	io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:357)
	io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:116)
	io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:137)
	java.lang.Thread.run(Thread.java:744)