The changes in #511 change what headers get set by PDFKit
and make all headers set by PDFKit lower-case.

However, the changes also affect code depending on headers
set by the app - which for Rack 2.x apps will be 'Content-Type'.

To address this, the code should check if Content-Type is present
and in that case use it, otherwise default to content-type.

As the code also sets this header (changing the content type
to 'application/pdf'), it should set the same header that
the original value is retrieved from.

So decide on the exact header name first, store it in the
'content_type_header' variable, and use it to index the
headers dict.

Fix #515.

As rack 3.x outright rejects mixed case headers, this test has to be marked
as pending - but passes (without the pending flag) with rack 2.x.