Change StringGetOwnProperty to produce the same strings that the lexer produces

[raskad]

This Pull Request fixes/closes #1459.

This changes StringGetOwnProperty to use String::from_utf16_lossy. This fixes the issue mentioned, but of course String::from_utf16_lossy is only a hack for the missing full UTF16 support. In my opinion it is the best way to go, as writing the codepoint as an escaped string may cause even more unexpected behavior.

[raskad]

Test result	master count	PR count	difference
Total	78,897	78,897	0
Passed	29,629	29,633	+4
Ignored	15,612	15,612	0
Failed	33,656	33,652	-4
Panics	2	2	0
Conformance	37.55%	37.56%	+0.01%

Fixed tests:

test/built-ins/RegExp/prototype/exec/u-captured-value.js [strict mode] (previously Failed)
test/built-ins/RegExp/prototype/exec/u-captured-value.js (previously Failed)
test/built-ins/Array/prototype/concat/Array.prototype.concat_spreadable-string-wrapper.js [strict mode] (previously Failed)
test/built-ins/Array/prototype/concat/Array.prototype.concat_spreadable-string-wrapper.js (previously Failed)