Mime-based content negotiation is easier to read

It's still based on a big regular expression, but hopefully the comments and
wording make it a little less scary for someone scanning through the code.

(still no idea what `String#gsub(/\b/, '')` does, though)
Remove a mysterious zero-width replacement

As far as I can tell, it replaced zero-width breakers with zero-width
replacement (that is, it seemed to have no effect).

No tests failed when I removed it. If it does do something, maybe a test or
a comment would be handy to explain the intention.

It was introduced in commit f7728b88c8cb33e5a783f7a779c761196d697343 -- the
original implementation.
Merge branch 'readable_content_negotiation' of https://github.com/rhunter/grape