add filter to strip out WWW-Authenticate headers in responses when request has "X-Requested-With: XMLHttpRequest" header to avoid browser BASIC auth dialogs.
extract method from anonymous class for testing
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add basic sanity check of omission logic
tidy stupid default param name that annoys me
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Use user-agent instead of X-Requested-With to determine if WWW-Authenticate should be omitted
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Merge pull request #299 from sonatype/NEXUS-6189-disable-www-authenticate

REVIEW [NEXUS-6189] disable www authenticate