IN operator now returns false if string indexer is used to search for property and returns null

[arekdygas]

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Description

Fixes #104.

When IN operator is used to search for property in an object and that object doesn't have the property, but has string indexer, then the indexer is used to look for property. Before fix, even if indexer returned null, i.e. "doesn't exist", IN returned true.

A few additional tests for IN operator were added as well, just to be sure if this change didn't break anything.

[CLAassistant]

All committers have signed the CLA.

[FlorianRappl]

LGTM - thanks a lot!