Prevent cache pollution by storing only the factories

[philwebb]

Update SpringFactoriesLoader so that the cache stores only the factories and not the complete loader.

Prior to this commit, if a cache entry was added with the thread conect classloader, the loader instance would a added and the classloader stored. If the thread context classloader subsequently changes, and a call is made to forDefaultResourceLocation with null for the classloader, the cached entry would be used which contains the older classloader.