Update the configurability of interface objects' length.
Check for LenientThis when testing attribute getters.
Test for calling attribute setters on an incorrect object.
Merge pull request #87 from Ms2ger/configurable-interface-object-length

Update the configurability of interface objects' length.
Merge pull request #89 from Ms2ger/lenientthis

Support [LenientThis] extended attribute (fixes #88).