Don't assume there's a Content-Type header

Fixes https://app.getsentry.com/sentry/sentry/group/16258585/

Author: mattrobenolt

src/sentry_github/plugin.py

@@ -76,7 +76,7 @@ def create_issue(self, request, group, form_data, **kwargs):
         except Exception, e:
             if isinstance(e, urllib2.HTTPError):
                 msg = e.read()
-                if 'application/json' in e.headers['Content-Type']:
+                if 'application/json' in e.headers.get('Content-Type', ''):
                     try:
                         msg = simplejson.loads(msg)
                         msg = msg['message']