Interpret empty server tag values as missing tag values.

As required by https://ircv3.net/specs/extensions/message-tags

src/ircmsgs.py

@@ -73,7 +73,12 @@ def parse_server_tags(s):
             server_tags[tag] = None
         else:
             (key, value) = tag.split('=', 1)
-            server_tags[key] = unescape_server_tag_value(value)
+            value = unescape_server_tag_value(value)
+            if value == '':
+                # "Implementations MUST interpret empty tag values (e.g. foo=)
+                # as equivalent to missing tag values (e.g. foo)."
+                value = None
+            server_tags[key] = value
     return server_tags
 def format_server_tags(server_tags):
     parts = []

test/test_ircmsgs.py

@@ -147,6 +147,14 @@ class IrcMsgTestCase(SupyTestCase):
         self.assertEqual(m.args, ('me', 'Hello'))
         self.assertEqual(str(m), s + '\n')
 
+        s = '@foo=;bar=baz;qux= ' \
+            ':nick!ident@host.com PRIVMSG me :Hello'
+        m = ircmsgs.IrcMsg(s)
+        self.assertEqual(m.server_tags, {
+            'foo': None,
+            'bar': 'baz',
+            'qux': None})
+
     def testTime(self):
         before = time.time()
         msg = ircmsgs.IrcMsg('PRIVMSG #foo :foo')