Add Buffer to structured-clone-test and key-type-test

[vweevers]

While writing docs I realized there's a gap in the tests. It doesn't test what happens to a Buffer when you get it with asBuffer: false (should be a Uint8Array if a value, and a ArrayBuffer if a key).

[vweevers]

In addition, there should be a test to put an ArrayBuffer and get it with asBuffer: true.