Expected ParseError for pure ruby variant

[bjfish]

Steps to reproduce:
ruby -rjson/pure -e 'p JSON.parse("{")'

Expected:
unexpected token at '{' (JSON::ParserError)

Actual:
{}

[eregon]

See oracle/truffleruby#1570 (comment) for the original issue (TruffleRuby uses json/pure currently).

So this is an issue of the pure version accepting input that the C-ext does not accept.

[marcandre]

Duplicate of #314