(PE-6019) Encode string request body per Content-Type header

This commit encodes a request body string per the content of the
Content-Type header.  In the event that no charset is specified in the
Content-Type header, the charset is set to UTF-8.  Previously, the
request body string was always encoded to ISO-8859-1 and was not
necessarily in sync with the value of the Content-Type header.
(PE-6019) Addressed PR feedback

This commit contains two changes:

* Revert the Pascal-casing of the `Accept-Encoding` header added to
  async.clj

* Use `.toString` method on the `Content-Type` class to handle the
  serialization of the `Content-Type` header value
Merge pull request #17 from camlow325/maint/master/PE-6019-encode-request-body-string-per-content-type

(PE-6019) Encode string request body per Content-Type header