Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=51400

Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=51400
Avoid known bottleneck in JVM when converting between Strings and bytes by always providing a Charset rather than an encoding name.
Based on a patch by Dave Engberg.

Note: There are some Charsets that the JVM must support and in those cases Charset.forName(String) should not throw an exception.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1138019 13f79535-47bb-0310-9956-ffa450edef68