* fix: support non-lower-case Content-Type header provided by app

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.

* rspec: add tests for mixed case Content-Type header support

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.

* rspec/content-type: make mixed case test pending only on Rack>=3.0.0

* By calling `IO.popen` with an Array of command arguments
  (ex: `['ls', '-l', ...]`) it runs the command as a separate process
  instead of running it in a sub-shell as a shell command. This prevents
  any arbitrary command injection or env variable interpolation, without
  needing complex shell-escaping logic.
  https://ruby-doc.org/core-3.1.2/IO.html#method-c-popen
* Changed `Configuration#executable` to return a String or an Array for
  when xvfb mode is enabled.
* Changed `PDFKit#command` to return an Array of command arguments for
  `IO.popen`.
* Removed argument quoting logic as it's not necessary when calling
  `IO.popen` with an Array of arguments.
* Rewrote some specs to test if the command's Array of arguments contains
  specific argument values.
* Added a custom RSpec `contain` matcher for testing if an expected Array
  exists within another Array.