Fix exchange_code_for_access_token.

Fixes "instagram.oauth2.OAuth2AuthExchangeError: You must provide a client_id" error

Reference: facebookarchive#249. All credit goes to yousong.

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", headers=headers, body=data)
         parsed_content = simplejson.loads(content.decode())
         if int(response['status']) != 200:
             raise OAuth2AuthExchangeError(parsed_content.get("error_message", ""))