specify urlencoded Content-Type (exchange_for_...)

In my situation Instagram was not able to read post content. And reason was content-type.

Author: Hossein Amin

instagram/oauth2.py

@@ -109,7 +109,8 @@ def exchange_for_access_token(self, code=None, username=None, password=None, sco
         data = self._data_for_exchange(code, username, password, scope=scope, user_id=user_id)
         http_object = Http(disable_ssl_certificate_validation=True)
         url = self.api.access_token_url
-        response, content = http_object.request(url, method="POST", body=data)
+        headers={"Content-Type":"application/x-www-form-urlencoded"}
+        response, content = http_object.request(url, method="POST", body=data, headers=headers)
         parsed_content = simplejson.loads(content.decode())
         if int(response['status']) != 200:
             raise OAuth2AuthExchangeError(parsed_content.get("error_message", ""))