Compound filters

I think there is a gap in the specification: it is not explicitly called out how compound filter values should be interpreted. Given `/aura/tracks?filter[title]=Foo&filter[artist]=Bar` should the server send all tracks with title matching "Foo" AND artist matching "Bar"? This would be the most intuitive behavior, but it's not specified.

Should clients be provided with a way to specify they want compound filters to be interpreted with an OR operator instead?

Another example: `/aura/tracks?filter[artist]=Bar&filter[artist]=Baz`. Would this keep the first value? Last value? Array of both? Assuming array, if we assume an array of values is always interpreted with an AND operator, then this specific query makes no sense... (unless wildcards are supported, but that's another issue). But with an OR operator available, this could be useful as it would allow clients to specify a number of desired values with one request.