Update ClassLoaderFilesResourcePatternResolver to support servlet
resources when it's being used with a WebApplicationContext.

Prior to commit 918e122d a `ResourceLoader` was not added to the
`ApplicationContext`, meaning that servlet resources could be found by
virtue of the protected `getResourceByPath()` method. Following commit
918e122d, the context `ResourceLoader` is set, meaning that all calls
to `getResource` delegate to the `ResourceLoader` and the
`ApplicationContext` methods are not invoked. Since the devtools
`ResourceLoader` wasn't Servlet aware, servlet resources could not
be found.

Fixes gh-7752