Support userinfo with empty username but non-empty password

[mika-fischer]

URIs like foo://:pwd@host are parsed correctly, i.e. _parts contains the correct info. But both userinfo() and toString() first check if username is set and ignore the password if username is empty.

[rodneyrehm]

Neither 3.2.1. User Information nor the ABNF (below) indicates the username requirement, so this is a bug.

userinfo      = *( unreserved / pct-encoded / sub-delims / ":" )

The entire <username>:<password> scheme seems to be an unspecified semantic, even though 7.5. Sensitive Information is warning not to expose secret data in URLs.

Anyway, to fix your particular issue you'll want to look at URI.buildUserinfo() and its test. PR welcome :)