Convert between ints and floats when loading numbers

Fix loading of numbers. (#3)

The javascript firestore sdks can convert automatically between integer and floating types.
This updates the loading code to convert between ints and floats.
Based on https://github.com/googleapis/google-cloud-go/blob/27b5416a80e6a65ee9eebda55f06def51b9b7b88/firestore/from_value.go#L119