Simplify logic in ExpressionMatcher.Matches

[stakx]

This fixes #237.

The method's previous semantics are preserved, but note that it could be simplified to just:

return ExpressionComparer.Default.Equals(this.expression, value as Expression);
//                                                        ^^^^^^^^^^^^^^^^^^^

Nothing in ExpressionMatcher ensures that this.expression cannot be null. Therefore, the null special case could (and for correctness, probably should) be left entirely up to ExpressionComparer.Equals.

[stakx]

Closing this PR as I will re-submit its commit as part of a larger PR.