I have the following problem with JMeter.

I am using GWT (Google Web Toolkit) java framework for building web apps. GWT
encapsulates ajax http calls, sending ajax requests via POST to server.

Server always expects content-type: text/plain; charset=utf-8. That is problem
because JMeter's http sampler changes headers. Even if I have correct setting
for Content-type in the http request sampler node, JMeter changes this and sends
Content-Type: application/x-www-form-urlencoded instead. As I said this is
problem with GWT ajax calls. Is there any way how to make JMeter not to change
this header? I used HTTP Request Manager Config element but with no success.

As a workaround I manually changed the source code, edited
org.apache.jmeter.protocol.http.sampler.PostWriter and commented line, which
changes the header. However, there should be a cleaner way to do it.

Vladimir