Add a getter for keysToUpdate field in the Update class

[hwhh]

Hi,

I am currently preforming field level authentication via an ABAC policy, and want support for allowing certain users to update certain fields. The Update class is a good syntactic sugar class but currently I can use it as I can't validate the fields being updated.

Alternatively it would be nice (not sure if possible) to "stage" the update and return the result POJO(s), that would result in the update being applied and then the document being loaded into memory through, for example, findById