Skip static rules

When using Flask 2x the send_static_file function is a lambda and
crashes the extraction of the rules_by_endpoint function. There is
currently a PR that has a fix but it's been stale so I'm adding this as
a possible alternate fix.

The other fix is: jmcarp#230

flask_apispec/extension.py

@@ -103,6 +103,10 @@ def register_existing_resources(self):
             except ValueError:
                 blueprint_name = None
 
+            # Skip static rules
+            if name == 'static':
+                continue
+
             try:
                 self.register(rule, blueprint=blueprint_name)
             except TypeError: