Don't set the content-type when there is no payload

The HTTP Get operation actually set a content-type of 'application/json' when there is no payload. The request adds a content-length of 0 because there is no payload.

Web server like https://github.com/stoplightio/prism (mock of openapi) reject the calls because the content-type has nothing to do with GET operations.

Exemple:
GET http://127.0.0.1:4010/RESTMethod HTTP/1.1
content-type: application/json
accept: application/json
host: 127.0.0.1:4010
content-length: 0
Connection: close

Signed-off-by: Lionel Schiepers <[email protected]>

Author: Lionel Schiepers