Make HypermediaAutoConfiguration more lenient.

HypermediaAutoConfiguration didn't consider an @EnableHypermediaSupport 
annotation being present in the user configuration which could've caused 
it to be evaluated twice.

This is especially the case if both the auto-configuration for Spring 
HATEOAS and Spring Data REST kick in as Spring Data REST actively declares
@EnableHypermediaSupport. The double evaluation then causes injection
ambiguities as we now get multiple beans of e.g. LinkDiscoverers deployed.