Allow XMLHttpRequest to be mocked by test frameworks (e.g., jasmine-ajax; sinon)

Currently the local xhrHttp is defined at require-time.

Unfortunately, libraries for mocking out XMLHttpRequests that rely on replacing global.XMLHttpRequest with a mock do not play nicely with this - you've already gained a reference to the real XMLHttpRequest by the time they replace the global.XMLHttpRequest reference.

Will be submitting a pull request to fix.

Locate where the local xhrHttp reference is initialized at require time and where XMLHttpRequest is used. Check the existing test setup for browser HTTP requests, then verify that replacing global.XMLHttpRequest with a mocking library is respected when the request is made.

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