boa: Attributes created by JSON.parse cannot be modified

JSON.parse seems to create object whose attributes cannot be modified (but can be deleted).

Steps To Reproduce Checkout current master (87aea64c) start cli: ./target/debug/boa Enter

var a = JSON.parse('{"x":0}');
a.x=2;
a.x

The value a.x is still 0 instead of expected 2.

Build environment (please complete the following information):

• OS: Ubuntu
• Version: 20.04
• Target triple: x86_64-unknown-linux-gnu
• Rustc version: 1.43.1 (8d69840ab 2020-05-04)