:call-seq:
  make_headers(user_headers) -> Hash<String, String>
 
Generate headers for use by a request. Header keys will be stringified
using `#stringify_headers` to normalize them as capitalized strings.

The final headers consist of:
  - default headers from #default_headers
  - user_headers provided here
  - headers from the payload object (e.g. Content-Type, Content-Lenth)
  - cookie headers from #make_cookie_header

BUG: stringify_headers does not alter the capitalization of headers that
are passed as strings, it only normalizes those passed as symbols. This
behavior will probably remain for a while for compatibility, but it means
that the warnings that attempt to detect accidental header overrides may
not always work.
https://github.com/rest-man/rest-man/issues/599

[Parameters:]

  - user_headers (Hash) -- User-provided headers to include

[Returns]
  - (Hash<String, String>) -- A hash of HTTP headers => values