GraphQL Request Sampler sets incorrect Content-Type header of text/plain

**Przemek Wesołek** ([Bug 65646](https://bz.apache.org/bugzilla//show_bug.cgi?id=65646&redirect=false)):
JSON requests send by GraphQL sampler contain wrong Content-Type header:

Content-Type: text/plain

which instead should be

Content-Type: application/json

by default.

One can of course use HTTP Header Manager to set the correct value for all requests, but I think this is only a workaround, as:

* the default should not be invalid (text/plain Content-Type is invalid for conforming GraphQL server),
* when mixing GraphQL queries with "standard" HTTP Requests with other content types, adding HTTP Header Manager might require more test reorganization

So I'd suggest just setting application/json as the default content type for GraphQL Request Sampler

Severity: normal
OS: Linux