Add rawHeaders to HttpResponse

[fredrikhr]

As per the documentation from node.js (which is also used by got), HTTP headers from the response are (in most cases) merged together by joining multiple values from equal-named headers with ', '. (I.e. multiple x-some-header headers are combined to one header value with <value1>, <value2>, etc.).

In most cases that is fine as there is some special handling for some default headers (e.g. User-Agent (multiple values are discarded) and Set-Cookie (value is always an array).

However, I find myself in a situation where I need to handle multiple values for a header. And to makes matters even worse, I need to parse these headers and the syntax I am parsing contains ,. In such a case the intelligent header handling of got is not helpful.

For such purposes I propose that we add a new property rawHeaders to the HttpResponse type which is populated from the same-named property from the got Response object.

Because the got raw-headers are a little cumbersome to work with, I propose to also add a merging helper function that combines equal named headers into arrays and group them by lower-cased header names. That way you can easily get the best of both worlds with little effort (i.e. simply pass the raw-headers into the helper function), while not requiring extra work to be done if you don't need it (rawHeaders is already accessible in the got HTTP Response object).

[AnWeber]

Thank you for the detailed explanation. Fortunately, I would not have to know this detail yet.
You might want to add the rawHeaders to the copy function as well. I will have a look at that tomorrow, if it is really needed

[AnWeber]

Thank you. I have removed the rawHeaders on output of JSON (httpyac --json). I think here they are not needed. There is a similar place in vscode-httpyac. I'll remove that one too. But otherwise it should fit.