colons: Prevent error when space before is mandatory

In the rare case when the key before `:` is an alias (e.g. `{*x : 4}`),
the space before `:` is required (although this requirement is not
enforced by PyYAML), the reason being that a colon can be part of an
anchor name. Consequently, this commit adapts the `colons` rule to avoid
failures when this happens.

See this comment from Tina Müller for more details:
#550 (comment)

tests/rules/test_colons.py

@@ -256,3 +256,19 @@ def test_both_before_and_after(self):
                    '  property: {a: 1, b:  2, c : 3}\n', conf,
                    problem1=(3, 11), problem2=(4, 4),
                    problem3=(8, 23), problem4=(8, 28))
+
+    # Although accepted by PyYAML, `{*x: 4}` is not valid YAML: it should be
+    # noted `{*x : 4}`. The reason is that a colon can be part of an anchor
+    # name. See commit message for more details.
+    def test_with_alias_as_key(self):
+        conf = 'colons: {max-spaces-before: 0, max-spaces-after: 1}'
+        self.check('---\n'
+                   '- anchor: &a key\n'
+                   '- *a: 42\n'
+                   '- {*a: 42}\n'
+                   '- *a : 42\n'
+                   '- {*a : 42}\n'
+                   '- *a  : 42\n'
+                   '- {*a  : 42}\n',
+                   conf,
+                   problem1=(7, 6), problem2=(8, 7))

yamllint/rules/colons.py

@@ -92,7 +92,9 @@
 
 
 def check(conf, token, prev, next, nextnext, context):
-    if isinstance(token, yaml.ValueToken):
+    if isinstance(token, yaml.ValueToken) and not (
+            isinstance(prev, yaml.AliasToken) and
+            token.start_mark.pointer - prev.end_mark.pointer == 1):
         problem = spaces_before(token, prev, next,
                                 max=conf['max-spaces-before'],
                                 max_desc='too many spaces before colon')