Consider improving X-Client-Cert URI decoding to support '+' for spaces

While doing functional review for SERVER-39, I needed to URI-encode a certificate from the command line. I found a simple Python one-liner to do it, but instead of using %20 for spaces it used a plus. The Server wasn't able to successfully decode this cert, but url-decode function didn't throw an exception and instead I see an error about the PEM not containing any certificates.

We should consider supporting this character, which would mean that we'd need to use a different url-decode function that supports it.