We have two representations of a source. Once used for sorting, which
should not depend on the source's state, but solely on its static
information, like remotes. Another one used for error and informational
messages, which should properly inform about the exact state of the
source when the message is printed.

This commit makes the latter be the default implementation of `to_s`, so
that error and informational messages are more accurate by default.

https://github.com/rubygems/rubygems/commit/b5f2b88957