Add HEADERS variable to the Rook request environment

[alandipert]

Fixes #141

TODO:

• [x] NEWS
• [x] Reference documentation

[wch]

Some notes:

• The req object now includes an item req$HEADERS, which is a named character vector.
• The names are all lowercase, since the HTTP spec says that capitalization does not matter, and making it all lowercase makes it easier to consistently index into.
• The order of headers is not preserved, also in keeping with the HTTP spec.
• If multiple headers have the same name, as in:

  foo: 123
  FOo: 456

  then they will be collapsed into a single value (req$HEADERS[["foo"]] == "123,456"), separated by a comma, and preserving the order. This is also what the HTTP spec says.

[jeroen]

Thanks this is great! Thank you!

[jeroen]

Make sure you test this on CentOS 6 though, some old compilers have trouble with iterators.

[alandipert]

@jeroen thank you for the feedback, I'm glad you like it!

Since making the PR we've become somewhat ambivalent about adding this now, since the info was already available, albeit in an inconvenient form. We (@jcheng5 and @wch and I) discussed passing for now, and instead holding out for a possible overhaul of the httpuv API that could include it.

What are you using it for? Would this be really helpful to you now, or would you be willing to wait?

Thanks in advance for sharing your perspective.

[jeroen]

I would like to use it for unit testing http requests in the curl package. What's the reason not to include it? It seems like a trivial non-breaking change?

[alandipert]

Thanks for your input @jeroen, scratch what I said, we're going to move forward with this 😄 I'll do the CentOS 6 testing in the very near future.

[jeroen]

Just checked in my centos 6 vm and all seems good !

[alandipert]

@wch when you have a chance can you please look at the NEWS and documentation additions and let me know if they're good?