Avoid empty body failure on HEAD requests

HTTP specification states clearly that HEAD requests
should have no body, so we should not fail on these.

Change-Id: I9d2418f26c26b2a40f6bafaf86ed6640fbf83b0c

jenkins/__init__.py

@@ -531,6 +531,7 @@ class Jenkins(object):
         response.raise_for_status()
 
         headers = response.headers
+        if response.request.method != 'HEAD':
             if (headers.get('content-length') is None and
                     headers.get('transfer-encoding') is None and
                     (response.content is None or len(response.content) <= 0)):