Add test for issue #50: pipe a null expression into Should.
Modify Should to handle pipeline input in END block. Closes #50.

In case a null expression is provided in pipeline input, powershell doesn't invoke the PROCESS block of a function. However Should must act on such an expression. See #50 for details.
Use script block to pass input to Should Throw validation for Assert-VerifiableMocks.
Refactor null expression input into a separate test for Should.
Merge branch 'codito-issue-50'