fix: Firebase.Database.Snapshot.exists function does not exists

[nattster]

1. Map Firebase.Database.Snapshot.exists to correct native function
2. Return boolean result from .exists() directly. Returning { ctor: "False" } evaluates to True in elm 0.18.0

[mrozbarry]

@nattster have you tested this, and it's working as expected?

[jjant]

@mrozbarry I made the same change on my local package dir to fix this

[SidneyNemzer]

Just got a runtime error because Firebase.Database.Snapshot.exists maps to Native.Database.exists, but that's not actually a function. @mrozbarry it would be awesome if you could merge this!

[SidneyNemzer]

Just an update, we've merged this PR into our fork here: https://github.com/ucode/elm-firebase

We'll be happy to merge PRs there, if anyone else finds bugs to fix!