Move content type to a const

pypubsub.py

@@ -30,6 +30,7 @@ import plugins.ldap
 
 # Some consts
 PUBSUB_VERSION = '0.4.6'
+PUBSUB_CONTENT_TYPE = 'application/vnd.pypubsub-stream'
 PUBSUB_BAD_REQUEST = "I could not understand your request, sorry! Please see https://pubsub.apache.org/api.html \
 for usage documentation.\n"
 PUBSUB_PAYLOAD_RECEIVED = "Payload received, thank you very much!\n"
@@ -117,7 +118,7 @@ class Server:
                 subscriber.acl = await subscriber.parse_acl(auth)
 
             self.subscribers.append(subscriber)
-            resp.content_type = 'application/vnd.pypubsub-stream'
+            resp.content_type = PUBSUB_CONTENT_TYPE
             try:
                 resp.enable_chunked_encoding()
                 await resp.prepare(request)