Return true if a userinfo is present
This function returns true if this contains a userinfo.
assert( url_view( "http://jane%2Ddoe:pass@example.com" ).has_userinfo() );
Constant.
Throws nothing.
userinfo    = user [ ":" [ password ] ]
authority   = [ userinfo "@" ] host [ ":" port ]
true if a userinfo is present
has_password, encoded_password, encoded_user, encoded_userinfo, password, user, userinfo.