Support URL encoded username and password in basic authentication

[mmatczuk]

We handle encoding fine thanks to implementation in url.UserInfo::String

Decoding the credentials on the server-side:

• On the server-side, when you receive the Authorization header, you need to decode the Base64-encoded username:password to extract the original username and password.
• After decoding, if the username or password was URL-encoded, you need to decode it again to get the original values.

[mmatczuk]

I'm closing this issue in favor of #431