Support :mulitpart env key to env_for to force multipart input

[jeremyevans]

No description provided.

[bjeanes]

What do you think about this defaulting this to check if ENV['CONTENT_TYPE'] is "multipart/form-data" or starts with "multipart/"?

[bjeanes]

I have successfully implemented a proof-of-concept integration with this multipart key and opened a PR teamcapybara/capybara#2557.

It does seem to necessitate a change to the interfaces of the Driver#submit and Browser#submit methods, about which the Capybara maintainers may have reservations.

[jeremyevans]

What do you think about this defaulting this to check if ENV['CONTENT_TYPE'] is "multipart/form-data" or starts with "multipart/"?

That seems reasonable to me, so I've updated the PR to do that.