Use relative path for api.specs_url.

This resolves any issues with mixed content when running behind an HTTPS reverse proxy.
There is no valid use case for using absolute over relative here, and only causes additional issues.

Resolves #188

Author: jslay88

flask_restx/api.py

@@ -503,11 +503,11 @@ def endpoint(self, name):
     @property
     def specs_url(self):
         """
-        The Swagger specifications absolute url (ie. `swagger.json`)
+        The Swagger specifications relative url (ie. `swagger.json`)
 
         :rtype: str
         """
-        return url_for(self.endpoint("specs"), _external=True)
+        return url_for(self.endpoint("specs"))
 
     @property
     def base_url(self):