api: fixes and changes to always return content type in API

- few tweeks here and there

rhodecode/api/tests/utils.py

@@ -86,7 +86,8 @@ def build_data(apikey, method, **kw):
 
 def api_call(app, params, status=None):
     response = app.post(
-        API_URL, content_type='application/json', params=params, status=status)
+        API_URL, content_type='application/json', params=params, status=status,
+        headers=[('Content-Type', 'application/json')])
     return response