Add URI::Generic#decoded_#{user,password}

[jeremyevans]

URI::Generic#{user,password} return the encoded values, which are
not that useful if you want to do authentication with them.
Automatic decoding by default would break backwards compatibility.
Optional automatic decoding via a keyword to URI.parse would
require threading the option through at least 3 other methods, and
would make semantics confusing (user= takes encoded or unencoded
password?) or require more work. Thus, adding this as a separate
method seemed the simplest approach.

Unfortunately, URI lacks a method for correct decoding. Unlike in
www form components, + in earlier parts of the URI such as the
userinfo section is treated verbatim and not as an encoded space.
Add URI.#{en,de}code_uri_component methods, which are almost the
same as URI.#{en,de}code_www_form_component, but without the
special SP => + handling.

Implements [Feature #9045]