Actually fix unescaping of proxy auth credentials

- The current way is broken as it chokes while unescaping characters that are not allowed non-escaped in the userinfo URI component, such as "@" if the username is an email address.
- Being a nice Ruby citizen means not mutating the `@proxy_uri` which is an object that might have came from the user.

References #54 #56. Fixes #48

/cc @juhakaja @tumf