Fixed LambdaHelper for extension methods

Previously, LambdaHelper did not work properly for extension methods.
This meant that converting a lambda expression to statement or extracting lambda expression to method did not work correctly when the lambda was used as an extension method.

I added an unit test that shows the problem.

This commit also includes a fixed version of LambdaHelper.

All this fix does is skip the first argument if the method is being used as an extension method.