Move "realm" attribute to be on <http> element rather than <http-basic>.

This faciltiates reuse with other mechanisms (like Digest) whilst also 
moving towards the <http-auto-configure> element (which benefits from 
having shared configuration in <http> as opposed to mechanism-specific 
elements).

core/src/test/resources/org/springframework/security/config/http-security.xml

@@ -15,7 +15,7 @@ http://www.springframework.org/schema/security http://www.springframework.org/sc
         <form-login loginUrl="/j_spring_security_check" />
 
         <!-- Default basic auth configuration. Will create filter and entry point -->
-        <http-basic realm="NamespaceTestRealm" />
+        <http-basic/>
 
         <!-- Default logout configuration -->
         <logout logoutUrl="/j_spring_security_logout" logoutSuccessUrl="/" invalidateSession="true" />