fix(request) add application/json content-type header to POST/PUT requests
fix(request) add application/json content-type header to POST/PUT requests

This fix affects all POST/PUT requests. Gitlab currently ignores
the body of POST/PUT requests when the content-type header is not set.

Even though the response status is 200, the requests have no effect on
Gitlab.

plouc/go-gitlab-client,fudanchii/go-gitlab-client