Multiple Accept header fix

[tsmethurst]

This PR updates Accept content negotiation to iterate through multiple Accept headers to find possible negotiable formats, rather than just taking the first Accept header and ignoring the others.

[tsmethurst]

Hello! Can this be merged? It's still pretty useful imo :)

[daenney]

@thinkerou @appleboy Would it be possible to get some eyes on this one?

It's very easy to run into this situation. When using the Go and preparing a request people might easily call req.Header.Add("Accept", ...) multiple times instead of concatenating the values with a ,, and on the command line people are very likely to use -H "Accept: ..." repeatedly with cURL instead of building the string by hand.

Based on my reading of the RFC, any field that accepts a comma separated list can also be represented as the same field multiple times. Given Accept is a list, Gin should understand both.