Use null in MockServletContext for unknown mime types

MockServletContext.getMimeTypes now returns null if the Java Activation
Framework returns "application/octet-stream", which is the default
media type it returns if the mime type is unknown. This enforces the
contract for ServletContext.getMimeTypes (return null for uknown mime
types) but does mean "application/octet-stream" cannot be returned.

Issue: SPR-10334