Use port in Mattermost only if defined

When we generate a MM notification URL with default port:
>>> NotifyMattermost(token="secret", host="chat.example.com", port=443, secure=True).url()
'mmosts://chat.example.com/secret/?image=no&format=text&overflow=upstream'

And then try to send it:

```
ac=Apprise()
ac.add(NotifyMattermost(token="secret",
                        host="chat.example.com", port=443, secure=True).url())

ac.notify("testing")
```

It fails. In debug logs we can see:
`DEBUG:apprise:Mattermost POST URL:
https://chat.example.com:None/hooks/secret (cert_verify=True)`

Port is always included in the POST url, but if port matches default
value for given "secret" we might not have it defined.

So let's only include port in Mattermost URL if not None

apprise/plugins/mattermost.py

@@ -205,6 +205,10 @@ def send(self, body, title='', notify_type=NotifyType.INFO, **kwargs):
         # Set our user
         payload['username'] = self.user if self.user else self.app_id
 
+        port = ''
+        if self.port is not None:
+            port = ':{}'.format(self.port)
+
         # For error tracking
         has_error = False
 
@@ -215,8 +219,8 @@ def send(self, body, title='', notify_type=NotifyType.INFO, **kwargs):
             if channel:
                 payload['channel'] = channel
 
-            url = '{}://{}:{}{}/hooks/{}'.format(
-                self.schema, self.host, self.port,
+            url = '{}://{}{}{}/hooks/{}'.format(
+                self.schema, self.host, port,
                 self.fullpath.rstrip('/'), self.token)
 
             self.logger.debug('Mattermost POST URL: %s (cert_verify=%r)' % (