RequestSerializer: include Content-Type and Content-Length headers.

[cristiangreco]

RequestSerializer returns, among other things, the filtered http headers
of the handled request, that is: all the HTTP_* headers normally
provided by Rack with a filtering rule applied (this is useful if you
want to e.g. mask or transform the content of some headers).

Among the request headers injected by Rack into the request environment
there are two notable exceptions to the 'HTTP_' rule: both CONTENT_TYPE
and CONTENT_LENGTH are not prefixed with 'HTTP_'. (more on this on the
Rack SPEC doc [0] and the lint file [1]).

This pull request changes the headers filtering logic to take both
CONTENT_TYPE and CONTENT_LENGTH into account. RequestSerializer will now
return 'http_content_type' and 'http_content_length'.

As far as I can tell this is a backward-compatible change as we're just
returning a new pair of headers. No filtering rule should be broken by
this change as such rules are only applied to the filtered headers.

[0] https://rubydoc.info/github/rack/rack/master/file/SPEC
[1] https://github.com/rack/rack/blob/master/lib/rack/lint.rb

[cristiangreco]

@lawrencejones I think you've already hit this in one of our services. I realised this thing when discovering that we're not logging the content-type of requests we get in.

[cristiangreco]

@nickcampbell18 do you think we could cut a new release with this in? In that case, should I update the changelog in this PR?

[nickcampbell18]

Yes that's probably best. It's probably a minor bump because this is a feature rather than bug?

[cristiangreco]

I've pushed the updates to CHANGELOG to prepare for 2.1.0 (it also includes a PR from @jacobpgn 🎉).

@nickcampbell18 feel free to merge and publish if you want to.