Code and tests for get() and has() to explicitly handle missing input.

[markstos]

Usually get() and has() are called with explicit inputs, but they can also be
called with a variable name, and that variable can unexpectly undefined or null,
as @robludwig found and submitted a patch for.

This pull request builds on his work by adding test coverage and also covering has()
as well as get().

@lorenwest: Note that for has() I have the code return false on missing input
instead of throwing an exception like get() does.

The value of has() is that it works differently that get() does, precisely so people
can avoid a try/catch block around get().

Since has() is largely for avoiding try/catch, it made sense to have it avoid
throwing exceptions.

[lorenwest]

This looks great - thanks for the tests!