Fix function check in var method

[horacio-penya]

Rules like:

logic.run({var: 'toString'},{toString: 'hello'}) // expect "hello"
logic.run({var: 'values'},{values: [1,2,3]}) // expect [1,2,3]

return null because "hello".toString is a function, even if we want to get "hello" (a valid object, not a function)

This happens because we're checking for context[key] even when key is already implied in context because we have descended into it during the previous for loop.

[TotalTechGeek]

Hey, I think I'll merge this, but I might make some touch ups afterwards. I did some additional testing on my side and it appears good.

[TotalTechGeek]

Thanks for the PR @horacio-penya.

I've provided a few other minor tweaks and have pushed a new version of this package out as 1.3.7