(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
Update CHANGELOG for 0.2.7 release
lein-release plugin: preparing 0.2.7 release
lein-release plugin: bumped version from 0.2.7 to 0.2.8-SNAPSHOT for next development cycle
(TK-88) Removed errant build call from JavaClient's createClient()

This commit removes an unnecessary build() method call made on the
HttpAsyncClientBuilder in JavaClient's createClient() method.
Previously, the presence of this call would cause an extra
CloseableHttpAsyncClient object to be created.  Since close() was never
called on the extra client object, native file descriptor resources were
never freed even after the object fell out of scope.
Merge pull request #18 from camlow325/bug/master/TK-88-errant-client-build

(TK-88) Removed errant build call from JavaClient's createClient()
Updated changelog for 0.2.8 release
Merge pull request #19 from camlow325/maint/master/clj-http-client-0.2.8-release

Updated changelog for 0.2.8 release
lein-release plugin: preparing 0.2.8 release
lein-release plugin: bumped version from 0.2.8 to 0.2.9-SNAPSHOT for next development cycle