Fix hasThis lookup regression

Path lookups on keys named `<something>this` results in invalid lookups, returning values for other properties or `undefined`.

This patch fixes the lookup, adds a test, and, as an added bonus, makes the lookup faster.

The faulty behavior was introduced in b9d2369f3d0e601f1d5ddc05e5008e931bac9003, and has been part of all stable releases since 1.8.0.

/cc @stefanpenner 
afaik, lastIndexOf() has always been valid for strings.