Prevent removing parameter overrides on first schema generation

This is ground work for #264 (closed). While working on this issue, I noticed that generating the schema once caused a lot of declared filters to be missing on all the subsequent generations for as long as the server is running; it turns out a .pop() on a dict caused the overriden parameters to go missing.

The failing test on !759 (merged) demonstrates the issue: https://gitlab.com/arkindex/backend/-/jobs/571343657