Make sphericalMercator default in XYZ code

The default for XYZ layers is for a non-spherical mercator projection. It's not clear what this projection is, in the code or documentation (probably EPSG:4326?), but nevertheless, the only example I can find of a non-merc XYZ tile layer is the server used in the examples, which has been down for at least a few weeks.

Thus, this patch changes that default to true. Users can still set `sphericalMercator:false;` if the want to take the reins, but given the lack of servers that are both XYZ and EPSG:4326 (at least that I can find that are online), this seems like a reasonable default that'll be what most users are expecting.