Add support for multi-line header values

[valyala]

Currently fasthttp doesn't support multi-line header values. See https://github.com/kataras/iris/issues/190 for details.

Probably, multi-line header values must be supported or properly handled by fasthttp.

[illotum]

This requires some design decisions.

Current header parser stores subslices to original headers, thus we can't follow http example and merge multi-line headers into one of the value1; value2 kind. We could return all lines with the matching key, but that means a change in Peek signature (to [][]byte).

[erikdubbelboer]

I don't think we should change the Peek signature. What we can do is add another method called PeekMulti(key string) [][]byte. One issue is that it will always result in a heap allocation for the [][]byte. We could avoid that by making it PeekMulti(key string, reuse [][]byte) [][]byte where reuse gets reused for the return value.

[kirillDanshin]

Of course, we will not change existing API. +1 on *Multi, if we'll find anyone that still needs that feature 😀

[Fenny]

I think this issue can be closed #688