Auto merge of #451 - longwusha:master, r=jdm
[#411] Add some tests for default-initialized roots
1. At the current stage, I only completed the test code for JSVal; I need to know how to implement several other types of default values in order to complete all the test code.
2. The default value of JSVal's current implementation is undefined, so we should use the is_undefined() method to verify the result.
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