Make parameters order stable for multi-parameter URLs (#106)

Fixes #105

* Added intermittently failing test for #105
* Make parameter order stable for urls with multiple params

src/drf_yasg/generators.py

@@ -403,7 +403,7 @@ class OpenAPISchemaGenerator(object):
         queryset = getattr(view_cls, 'queryset', None)
         model = getattr(getattr(view_cls, 'queryset', None), 'model', None)
 
-        for variable in uritemplate.variables(path):
+        for variable in sorted(uritemplate.variables(path)):
             model, model_field = get_queryset_field(queryset, variable)
             attrs = get_basic_type_info(model_field) or {'type': openapi.TYPE_STRING}
             if getattr(view_cls, 'lookup_field', None) == variable and attrs['type'] == openapi.TYPE_STRING: