Updated method signature for FakeFS::FakeSymlink#respond_to?

* Old version only accepted one parameter, which was no longer correct
  as of Ruby 1.8.7 (or, for that matter, 1.8.6 - I can't tell when the second
  parameter was added to the signature).
* Test suite validates the method accepts both parameters and that
  the behavior is implemented correctly.  It also validates that the
  default values are resolved correctly.  Note that the change in
  behavior in Ruby 2.0.0 for the second parameter was implemented by
  modifying the meaning of the second parameter (from `include_priv`
  to `include_all`) rather than changing the default value (which
  remained `false`), but it is still better not to copy method
  signatures if possible.