Fixes toJSON() for Struct to correctly return a dom.Value instance

[desaikd]

Issue #751

Description of changes:

This PR works on fixing toJSON() for Struct to correctly return a dom.Value instance.

Solution:

• JSON.stringify() uses toJSON() if the object has that defined as per MDN:

If the value has a toJSON() method, it's responsible to define what data will be serialized. Instead of the object being serialized, the value returned by the toJSON() method when called will be serialized.

• Previously the toJSON() for struct was returning an object based on null object prototype which was giving incorrect results for instanceof when called inside the JSON.stringify replacer function.
• Now we are using Struct's prototype to for the return object on toSJON().

List of changes:

• Struct#toJSON() now creates an object based on Struct's prototype instead of null object's prototype.

Tests:

added a unit test to check for instanceof dom.Value inside JSON.stringify replacer function.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[zslayton]

🚀