String() does not set length on instance

[jasonwilliams]

Describe the bug
Calling new String('some text') does not set the instance

E.g

let a = new String("1234");
a.length;

Should return 4

To Reproduce
Put the above into a test.js file and run it, you should get 0

Expected behavior
it should return 4

Implementation is here:
Master: https://github.com/jasonwilliams/boa/blob/master/boa/src/builtins/string/mod.rs#L31-L49

The string passed in lives at args.get(0), you can call len() then convert it back into a JSValue by calling to_value() on it.
There's an example commented out here:
https://github.com/jasonwilliams/boa/blob/master/boa/src/builtins/string/mod.rs#L36-L37

 let a: String = from_value(args.get(0).expect("failed to get argument for String method").clone()).unwrap();
 this.set_field_slice("length", to_value(a.len() as i32));