Ensure response to HEAD request won't have message body

Ensure that Gunicorn won't try to use chunked transfer-encoding for responses
to a HEAD request, so that `Response.close` will not write a terminating
chunk. Responses to a HEAD request MUST NOT have a message-body.

The application is still responsible for ensuring no message body is actually
generated in response to a HEAD request.

gunicorn/http/wsgi.py

@@ -230,6 +230,8 @@ def should_close(self):
             return True
         if self.response_length is not None or self.chunked:
             return False
+        if self.req.method == 'HEAD':
+            return False
         if self.status_code < 200 or self.status_code in (204, 304):
             return False
         return True
@@ -287,6 +289,9 @@ def is_chunked(self):
             return False
         elif self.req.version <= (1, 0):
             return False
+        elif self.req.method == 'HEAD':
+            # Responses to a HEAD request MUST NOT contain a response body.
+            return False
         elif self.status_code in (204, 304):
             # Do not use chunked responses when the response is guaranteed to
             # not have a response body.