Make updateDocument mask default to true

[legenduzo]

Please consider making the mask in updateDocument(path, fields, mask = false) true instead of false.

By default the expectation of the end-user is that the fields specified in fields are updated and it comes as a rude shock if the whole document is overwritten and it's just their new update that is reflected.

Defaulting mask = true will be consistent with document updates, with the optional choice of overwriting the whole document by consciously setting it to false.