HTTPCLIENT-1466: FileBodyPart#generateContentType() ignores custom ContentType values

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1570525 13f79535-47bb-0310-9956-ffa450edef68
HTTPCLIENT-1467: MultipartEntityBuilder#addPart(FormBodyPart) made public

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1570527 13f79535-47bb-0310-9956-ffa450edef68