This is a private method, and not used since
3855d1d.

The reason for deprecation before removal is that the
method only exists so it can be overridden in subclasses
(d8ccbf3), so if subclasses
were overriding it and calling super, we need to warn them
about the removal.

Found by coverage testing.

The only call of this method was removed in
51b0c26.

This method was only added a couple years ago, when Digest support
was removed in 253fc5b (it previously
called Rack::Auth::Digest::Params::dequote). So it seems unlikely
there are external users, and therefore it should not need
deprecation.

Found by coverage testing.

…filenames

This regexp was already questionable because of the use of ^ and $.
It turns out this was no longer needed, since the parser earlier
in the same method handles the quotes, so the result is that this
was removing escaped quotes that were part of the filename.

Found by coverage testing.

Rack::MediaType#type handles empty string, since
1848851.  The change does
not appear to be deliberate, though.  An alternative approach
would be both of them raising an exception if provided an
empty string, but I think handling the empty string similar
to nil is friendlier.

Found by coverage testing.

Technically, this leaves 1 branch uncovered, but that is about
to be removed.