HTTPCLIENT-1503: setter for custom multipart subtypes

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1599553 13f79535-47bb-0310-9956-ffa450edef68
https://github.com/apache/httpclient/pull/12
Configurable charset for request and URI builders
Contributed by isabelabel <bel at jusbrasil.com.br>

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1599798 13f79535-47bb-0310-9956-ffa450edef68