Code generation should set appropriate Content-type header

[n14s]

When sending a request through yaade, the Contest-type header is automatically set, at least in case of application/json.
The generated code should also include the Content-type header, otherwise a command eg. shell wget might fail with an error of a missing Contest-type header.

[jonrosner]

we don't set automatic headers. You can always set the header in the "body" tab by clicking "Set Header" next to the content type dropdown.