Zend\Authentication\Adapter\Http::_challengeClient() should be public since one could need to call it this way:

<code>$auth->getAdapter()->challengeClient();</code>

for instance when the authenticated user tries to access a resource for which he is not granted by ACLs.
changed _challengeClient() as deprecated proxy
Coding standard fix (spaces and tabs)
Added unit test
Indentation cleanup
Seems PHP_CodeCoverage doesn't like @covers on protected methods.

ZendTest\Authentication\Adapter\HttpTest::testProtectedMethodChallengeClientTriggersErrorDeprecated
 PHP_CodeCoverage_Exception: Trying to @cover not existing method "Adapter\Http::_challengeClient".