Fix AssertionError: Status must be a string (#715)

Signed-off-by: Chris Marchbanks <[email protected]>

Author: csmarchbanks

prometheus_client/exposition.py

@@ -114,8 +114,8 @@ def prometheus_app(environ, start_response):
         params = parse_qs(environ.get('QUERY_STRING', ''))
         if environ['PATH_INFO'] == '/favicon.ico':
             # Serve empty response for browsers
-            status = '200 OK'
-            header = ('', '')
+            status = str('200 OK')
+            header = (str(''), str(''))
             output = b''
         else:
             # Bake output