Multiple auth method support

[szakharchenko]

Split WWW-Authenticate by challenge and handle digest-related properties only for digest method. Add tests to make sure Digest is chosen as the more secure variant.

[coveralls]

Pull Request Test Coverage Report for Build 66

• 23 of 25 (92.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.4%) to 73.364%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
digest-fetch-src.js	23	25	92.0%

Totals
Change from base Build 63:	1.4%
Covered Lines:	99
Relevant Lines:	120

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💛 - Coveralls

[devfans]

Thanks. I will look into it later

[tomboland-vocovo]

I think this would fix a problem I have in that if the server asks for basic auth, but we don't know ahead of time that basic auth is required, then this library is making a request with an authorization header like this:

'Basic username="admin",realm="",nonce="",uri="/theUrl.html",algorithm="MD5",response="89ba6226c789a01785b93b888b130941",nc=00000001,cnonce="ed652fece3410971adb80a587bd016d3"'

So, without this PR, it will be necessary to try with basic = true in the options first, catch a failure, or try node-fetch first, get the www-authenticate header and figure out whethe Basic or Digest is necessary before using digest-fetch. I think this PR will solve all these problems.