Don't send an empty HTTP header. (#1101)

This breaks strict middleware..

Signed-off-by: Nils O. Selåsdal <[email protected]>

Author: noselasd

prometheus_client/exposition.py

@@ -132,7 +132,7 @@ def prometheus_app(environ, start_response):
         elif environ['PATH_INFO'] == '/favicon.ico':
             # Serve empty response for browsers
             status = '200 OK'
-            headers = [('', '')]
+            headers = []
             output = b''
         else:
             # Note: For backwards compatibility, the URI path for GET is not