No fluent setter for usingDefaultExcludes in AbstractFileSet

[redzi]

This is not an issue but a proposal for a very minor change that would make the creation of FileSets easier.
There is no fluent way of setting the usingDefaultExcludes flag in AbstractFileSet.

Due to this inconvenience instead of a completely fluent way of creating a FileSet instance:

fileSet(new File("foo"))
          .prefixed("pfx")
          .include(includes)
          .exclude(exc)
          .usingDefaultExcludes(usingDefaultExcludes);

one needs to do it like this:

DefaultFileSet fileSet = fileSet(new File("foo"))
          .prefixed("pfx")
          .include(includes)
          .exclude(exc);

fileSet.setUsingDefaultExcludes(false);

The fileSet variable needs to be of type DefaultFileSet because the FileSet interface does not provide the setter.

[redzi]

PR with the addition of the fluent setter: #312