Allow OPENAPI_URL_PREFIX to be '/'

Author: fbergroth

flask_rest_api/spec/__init__.py

@@ -66,7 +66,7 @@ def register_field(self, field, *args):
 
 def _add_leading_slash(string):
     """Add leading slash to a string if there is None"""
-    return string if string[0] == '/' else '/' + string
+    return string if string.startswith('/') else '/' + string
 
 
 class DocBlueprintMixin: