Revert "Return an actual boolean from xml_http_request?"
Reason: This commit changes code that was committed some year and a half ago. The original code is an ordinary predicate that delegates straight to a boolean operator with no further unnecessaru adorments, as clearly explained in #5329. This change also may confuse users who may now believe they can rely now on singletons, while predicates in Rails rely on standard Ruby semantics for boolean values and guarantee no singletons whatsover. This reverts commit 6349791.