Format messages in validates_format_of

I just submitted a patch to substitute the attribute value into the
message in validates_format_of, the same way validates_length_of,
validates_inclusion_of, and validates_exclusion_of already do.

If that seems useful, please take a look. Thanks.

Though there's one +1 on it, it doesn't seem like there's much
interest in that patch.

I'm currently working on adding consistent message formatting for all
the validations (where it seems to make sense). To clarify, the change
passes the rejected value to the message's % method (in addition to
what's already passed for those validations that already plug in mins,
maxes, what-have-you), so you can do something like this:

  validates_length_of :login, :min => 3, :message => "has to be at
least $d characters. '$s' isn't long enough."

Besides the fact that we need it in one validation for our app, the
main motivation was to get the other validations consistent with
validates_inclusion_of and validates_exclusion_of. (I was mistaken
earlier when I said validates_length_of already does this too.)

Will update the ticket once all validations are formatting the same way.

Oops ... I didn't notice the patch from that ticket was already
committed[1]. Great.

So I've entered a new ticket[2] with a patch adding the same feature
to the other validations, plus doc updates. I just added examples (or
modified the existing examples). It seemed too small a feature to
explain in wordy detail.

Please take a look. Thanks.