Fix KeyError 'content-type'

Some responses (like delete calendar event) don't include a Content-Type header.

Author: Kurt De Vos

microsoftgraph/client.py

@@ -613,7 +613,7 @@ def _request(self, method, url, headers=None, **kwargs):
 
     def _parse(self, response):
         status_code = response.status_code
-        if 'application/json' in response.headers['Content-Type']:
+        if 'application/json' in response.headers.get('Content-Type', ''):
             r = response.json()
         else:
             r = response.content