Spring use constants BEARER and BASIC instead of String literals

I think using constant BEARER instead of "Bearer ", and constant BASIC instead of "Basic " is better to reduce the risk of mistakes and increase reusability.

Comment From: pivotal-cla

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Comment From: bclozel

"Bearer " and "Basic " are not HTTP header names, so it doesn't make sense to expose them as public constants on HttpHeaders. I'm declining this PR as a result. Thanks!