get_request_uri broken due to site stripping

[chrysn]

When doing _find_child_and_pathstripped_message, the site strips the opt.uri_path options (as by design), but the stored _original_request_uri does not wind up used in Message.get_request_uri(), so that a server can't accurately tell which resource is being accessed.

I suggest hot-fixing this now and revisiting when CRIs are fully usable.

Discovered while answering https://stackoverflow.com/questions/75185387/how-to-get-the-name-of-the-uri-path-that-coaps-client-is-writing-to-in-resource