Mock ParameterFilter fix

ParameterFilter blocks are no longer forced to return a boolean value.  Since any value can be implicitly cast to Boolean anyway, this wasn't adding any value.

Also, the initial call to Test-ParameterFilter which would enforce this condition could sometimes wind up throwing errors due to null reference exceptions, in cases where the filter assumes that a mandatory parameter must not be null.
Examples update

Examples updated to avoid possible null reference exceptions in the parameter filter (calling a method on an optional parameter.)
Merge pull request #219 from dlwyatt/MockFix

Mock ParameterFilter fix