Prevent InvalidAccessError when using a synchronous XHR in browser

[sebcanonica]

Changing the responseType only when necessary, as setting it too liberally can cause an exception, like when using a synchronous request in a browser context.
Issue #61

[fatso83]

If this is supposed to fix an existing issue, could you add a test to lib/fake-xhr/index.test.js called something like describe("issue 61 - InvalidAccessError on synchronous XHR in browser", ...) that throws when your fix is not added? I am guessing you have a test case in #61 already.

[sebcanonica]

Ok, I have added a mockist test for this issue.
Do you have a way to execute the test suite in a browser?
I tried with mochify like in sinon main repo, but the suite keep getting stuck randomly.
When run in a browser, the disabled test "loads resource synchronously" fails even before the asserts because of the original problem.

[fatso83]

Thank you for that! We should definitively have a mockito suite running Chrome headless, like in the other sibling projects. We'll look into it.

[fatso83]

Based on the remarks from #69 I expect this to pass, even though it's hasn't run with the new mockito script on Travis.

Thanks a lot!