Enabled quirks mode on JSON.parse, fixes broken test in af9caae

It turns out that ActionPack depends on the decoder to parse JSON
"fragments" (e.g. '"a string"', '1', 'null', etc), so we need to
enable quirks mode on JSON.parse. Also added coverage on the decoder
side to prevent regression.
Merge pull request #12207 from chancancode/fix_json_load

Enabled quirks mode on JSON.parse, fixes broken test in af9caae