Adjust EnumerableOwnPropertyNames to use all String type property keys

[raskad]

This Pull Request fixes/closes #1501.

It changes the following:

• Adjust EnumerableOwnPropertyNames to use all String type property keys

This looks different from the spec, because the meaning of String in 4.a: If Type(key) is String, then is different than our internal PropertyKey enum types String, Symbol and Index. Our internal Index type is a String type in the context of the spec.
This PR is only a fix for this specific function. We should change our [[OwnPropertyKeys]] to be spec compliant, but I think we should wait with that until #1354 is merged, because [[OwnPropertyKeys]] is defined multiple times for different object types.

[raskad]

The broken test is actually broken and should not have passed before ;)

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,897	78,897	0
Passed	31,283	31,287	+4
Ignored	15,612	15,612	0
Failed	32,002	31,998	-4
Panics	2	2	0
Conformance	39.65%	39.66%	+0.01%

Fixed tests:

test/built-ins/Object/keys/15.2.3.14-5-14.js [strict mode] (previously Failed)
test/built-ins/Object/keys/15.2.3.14-5-14.js (previously Failed)
test/built-ins/Object/keys/15.2.3.14-3-4.js [strict mode] (previously Failed)
test/built-ins/Object/keys/15.2.3.14-3-4.js (previously Failed)
test/built-ins/Object/keys/15.2.3.14-3-3.js [strict mode] (previously Failed)
test/built-ins/Object/keys/15.2.3.14-3-3.js (previously Failed)

Broken tests:

test/built-ins/Object/keys/15.2.3.14-5-11.js [strict mode] (previously Passed)
test/built-ins/Object/keys/15.2.3.14-5-11.js (previously Passed)

[Razican]

Looks very good, nice catch!