Fix broken test code, that was made apparent when adding license headers to the text files. The old helper method did not test for the number of files in a directory, but rather the number of bytes in a directory. With only empty files it reported 0 (bytes), even though there were files available. This also pointed out an error in the test method itself, which was testing for the wrong condition.

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1525434 13f79535-47bb-0310-9956-ffa450edef68
Add an IT that covers a similar case to the one in the unit test I just fixed. I used this to test the proper behavior of simultaneous includes and excludes in a fileset.

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1525437 13f79535-47bb-0310-9956-ffa450edef68
Add missing license headers.
This plugin now passes the checks done by RAT.

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1525440 13f79535-47bb-0310-9956-ffa450edef68