generalise getName/getFilename into getContentDispositionValue

This pull request aims to resolve "this is the same code as above, but with a different token; generalise" FIXME as well as the "first character truncated from filename in Content-Disposition" issue reported in http://www.mail-archive.com/sword-app-tech@lists.sourceforge.net/msg00337.html

It uses http://commons.apache.org/proper/commons-fileupload/apidocs/org/apache/commons/fileupload/ParameterParser.html

Please note that a semicolon is used as a separator but http://svn.apache.org/viewvc/commons/proper/fileupload/trunk/src/test/java/org/apache/commons/fileupload/ParameterParserTest.java?r1=1455469&r2=1455470&pathrev=1515823& suggests that a comma might be a legitimate separator per https://issues.apache.org/jira/browse/FILEUPLOAD-139