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 come from the user.

References:
- ac161c75f37073701c59c80b3e924a3a6aec8393
- b9031bf43d78cfc946fc7f9a55c4931e964fb05c
Merge pull request #59 from mislav/proxy-credentials-unescaping

Actually fix unescaping of proxy auth credentials