UTF-8 charset for API request payload.

Charset fix for below exception.

2013-11-20 23:06:50,941 DEBUG [qtp17541138-764] (org.zendesk.client.v2.ZenDesk) Request PUT ......
2013-11-20 23:06:51,124 DEBUG [New I/O worker #5] (org.zendesk.client.v2.ZenDesk) Response HTTP/422 Unprocessable Entity
{"error":"Unprocessable Entity","message":"Server could not parse JSON"} 
2013-11-20 23:06:51,125 WARN  [qtp17541138-764] (org.eclipse.jetty.servlet.ServletHandler)  
org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.zendesk.client.v2.ZenDeskResponseException: Unprocessable Entity
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:948)
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:827)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:812)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
	at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:696)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:526)
	at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1110)
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:453)
	at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1044)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
	at org.eclipse.jetty.server.Server.handle(Server.java:442)
	at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:279)
	at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:213)
	at org.eclipse.jetty.io.AbstractConnection$1.run(AbstractConnection.java:505)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:601)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:532)
	at java.lang.Thread.run(Thread.java:744)
Caused by: org.zendesk.client.v2.ZenDeskResponseException: Unprocessable Entity
	at org.zendesk.client.v2.ZenDesk$3.onCompleted(ZenDesk.java:764)