Fix default value for a header

This code specified None as the default value, but it would raise an
exception if that was ever returned.  An empty string causes the code to
work as intended.

This came up while I was writing a unit test for another bug fix.

Change-Id: I658bb8a9b5124f281988fb60b645182ea0ccf99f
Related-bug: #1231524

glanceclient/common/http.py

@@ -248,7 +248,7 @@ class HTTPClient(object):
 
         resp, body_iter = self._http_request(url, method, **kwargs)
 
-        if 'application/json' in resp.getheader('content-type', None):
+        if 'application/json' in resp.getheader('content-type', ''):
             body = ''.join([chunk for chunk in body_iter])
             try:
                 body = json.loads(body)