Refactor headers

[ixti]

No description provided.

[ixti]

Ready for review
//cc @sferik @tarcieri

[ixti]

Notice, there's a backward incompatible change. Behavior before this PR:

request[:accept] = 'text/plain'
request[:accept] = 'application/json'
request[:accept] # => ['text/plain', 'application/json']

Behavior after this PR:

request[:accept] = 'text/plain'
request[:accept] = 'application/json'
request[:accept] # => 'application/json'

In order to get same result as before this PR one would have to:

request[:accept] = ['text/plain', 'application/json']

# or

request.headers.add :accept, 'text/plain'
request.headers.add :accept, 'application/json'

[sferik]

I’m 👍 on the new interface. = should always overwrite. The current behavior seems wrong to me.

[sferik]

Overall, this looks good. I’m 👍 to merge.

[tarcieri]

+1 on the current = behavior being wrong. This is some great work, thank you! 👍