Use 'xyz' as scheme in metadata

[hannesj]

As getTile swaps the z-coordinate from tms to xyz

Fixes #65

[jingsam]

ping @mapsam

[mapsam]

@flippmoke can you confirm this is 👍? Since you just want to confirm since you recently updated the mbtiles specification.

[jingsam]

ping @mapsam

[mapsam]

@jingsam thanks for the ping again- I've been hesitant to merge this only for the sake of being unsure about the downstream ramifications of switching this over. Should/does the scheme field represent the user expectations of the tileset (in which case zxy makes sense), or does it represent the internals of the MBTiles format (in which tms is the spec)? I'm unsure of how downstream tools may respond to the field, any experience here?

[hannesj]

Based on my understanding, the metadata should reflect what the tilelive api responds with. The internal storage within the mbtiles is just a implementation detail for the tilelive interface. I created this PR for us to be able to render tiles properly with https://github.com/mojodna/tessera

[jingsam]

@mapsam I agree with @hannesj . scheme should be tms in metadata table to represent the internal of the MBTiles format. However, when users retrieve information form getInfo interface, the scheme should represent tile service scheme, which always be xyz.

[mapsam]

@jingsam @hannesj 👍 let's do it.

[jingsam]

@hannesj I think putInfo should also write scheme: tms into metadata table. Would you mind revise your PR and add some test?