Rework Request.headers

[kgriffs]

To make the Request.headers property more useful for the common use case of proxying requests to an upstream service, let's return a list of tuples instead of a dict. This is obviously a breaking change, but easily worked around by the caller simply instantiating a dict with the results.

Furthermore, we should take the opportunity to return capitalized headers vs. all-upper-case (for parity with #578. We can use a lookup dict for standard/common headers to ensure this does not cause a significant performance degradation, and only fall back to a slower method for non-standard headers.

All the docs, including the tutorial, will need to be checked for any references to the 1.x behavior that may need to be updated for this breaking change.

[steven-upside]

@kgriffs I was thinking about taking this one on, but this seems closely related to #578 which just moved to in-progress. Would it be better to pick something else up to avoid conflicting changes?