Get raw headers from request

[alandipert]

Currently, callback functions registered via call or onHeaders can only receive the "Rook-style" request headers ("HTTP_ Variables" in the Rook README).

It would be useful if there was a way to receive the raw headers, before they were munged to become variable names, perhaps as a list of pairs of strings. That way, headers could be inspected without accounting for Rook's renaming of them.

Interested parties so far include @wch and @jeroen.

This capability raises various questions.

Questions

1. What value should be used to represent raw headers?
   1. Shouldn't be an env because the same header might appear multiple times. This is valid in HTTP under certain circumstances
2. How should a handler be registered so that httpuv knows to pass it raw headers? And, how should that value be passed? Possibilities include:
   1. New callRaw and onHeadersRaw app members that pass the handler a second argument, the raw headers value. Using in conjunction with call/onHeaders respectively is an error.
   2. Always add the raw headers as an attribute of the Rook req object passed to call/onHeaders
      1. Problematic because attributes can contribute to object equality. Might not be backward-compatible?
      2. Introduces overhead we don't want to pay all the time?
   3. Add an addRawRequest = TRUE (default FALSE) member to app. This would be the only member of app so far that's not a function. Causes raw header value to be added to all reqs as an attribute.
   4. Like above, but add the addRawRequest = TRUE optional argument to startServer/startPipeServer instead of as app member.
   5. Add req$headers
      1. Should this be an addition to Rook first, then httpuv?

[wch]

I think the most straightforward way to go is to use a named list, like req$headers, and to provide it unconditionally. Using a named list allows us to preserve the header order, and also to have multiple HTTP headers with the same name. This will be a feature of httpuv; we don't need to go change Rook.

@jeroen You had asked before for the "exact headers" from the request. Would a named list as described above be sufficient?

[alandipert]

@wch and I took a first cut at an implementation and made a PR: #143

It adds HEADERS, a named character vector, to the Rook request env.

We have some code that uses httpuv as a proxy frontend with a curl backend, and this change would simplify the conversion of Rook requests to parameterized curl handles.

@jeroen, would it suit your needs as well? We're curious to hear if it would work for you too, or if you otherwise have any feedback.