Validating object keys in update()

[schmidt-sebastian]

This adds FieldPath validation to update( {} ) and makes sure no invalid characters are used as field paths.

[codecov]

Codecov Report

Merging #65 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #65   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          11     11           
  Lines        1478   1479    +1     
=====================================
+ Hits         1478   1479    +1

Impacted Files	Coverage Δ
src/document.js	100% <100%> (ø)	⬆️
src/write-batch.js	100% <100%> (ø)	⬆️

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