Fix RFC 8259 test cases around unescaped characters

[bowbahdoe]

Currently the following test files that should be rejected are accepted.

• n_string_unescaped_newline.json
• n_string_unescaped_tab.json
• n_string_unescaped_ctrl_char.json

Its not horrible imo since its slightly more tolerant and won't reject valid input, but worth fixing (I think)