Description:
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If a REST API returns valid HTTP status code 207 Multi-Status the fetch() method throws an exception:
Invalid auth/bad request (got a 207, expected HTTP/1.1 20X or a redirect)

In fact 207 IS HTTP/1.1 20x series code but fetch() does not recognize it.

Test script:
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It cannot be easily tested because you need an Oauth server / REST API.

Expected result:
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Expected result is that fetch() would not throw an exception if HTTP status code is from 20x series.

Actual result:
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Invalid auth/bad request (got a 207, expected HTTP/1.1 20X or a redirect)