Fix buffer leak in DatagramUnicastTest caused by incorrect usage of CompositeByteBuf

Motivation:

Due incorrect usage of CompositeByteBuf a buffer leak was introduced.

Modifications:

Correctly handle tests with CompositeByteBuf.

Result:

No more buffer leaks
Directly write CompositeByteBuf if possible without memory copy. Related to [#2719]

Motivation:

In linux it is possible to write more then one buffer withone syscall when sending datagram messages.

Modifications:

Not copy CompositeByteBuf if it only contains direct buffers.

Result:

More performance due less overhead for copy.
HTTP Content Decoder Cleanup Bug

Motiviation:
The HTTP content decoder's cleanup method is not cleaning up the decoder correctly.
The cleanup method is currently doing a readOutbound on the EmbeddedChannel but
for decoding the call should be readInbound.

Modifications:
-Change readOutbound to readInbound in the cleanup method

Result:
The cleanup method should be correctly releaseing unused resources
Correct javadoc typo in IdleStateHandler example code