Add missing Javadoc and throws declaration of unchecked exception UnsupportedCharsetException.

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1524452 13f79535-47bb-0310-9956-ffa450edef68
Ctor should throw unchecked UnsupportedCharsetException, not unchecked UnsupportedEncodingException. The Javadoc is still correct.

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1524454 13f79535-47bb-0310-9956-ffa450edef68
Add missing @throws in Javadoc.

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk@1524455 13f79535-47bb-0310-9956-ffa450edef68