Update MockServletContext to MediaTypeFactory

This commit changes the `MockServletContext.getMimeType` method to use
`MediaTypeFactory` instead of JAF. It also adds a `addMimeType(String,
MediaType)` method to customize the mime types returned from said
method.

Issue: SPR-14908

spring-webmvc/src/test/resources/org/springframework/web/servlet/resource/test/foo.bar

@@ -0,0 +1 @@
+h1 { color:red; }