fix: make fetch promise patching safe

[marclaval]

If the fetchPromise is a ZoneAwarePromise, then patching it creates infinite loop of .then().

[vicb]

Please add a test

[marclaval]

For the test, the only way I found to reproduce this issue is to load a window.fetch polyfill (e.g. https://github.com/github/fetch) in browsers which do not support this feature: http://caniuse.com/#feat=fetch

With the polyfill and without this fix, Safari 10 errors with RangeError: Maximum call stack size exceeded., and IE11 with Error: Out of stack space.