SWIFT-621 Provide default values for WriteModel options associated values

[patrickfreed]

SWIFT-621

I looked through the rest of our enums (ty jazzy) and didn't see anything else that needed this treatment besides WriteModel.

[kmahar]

lgtm, though it just occurred to me we can probably remove options: nil from a lot of places in the bulk write tests now?

[patrickfreed]

good point, done

[codecov-io]

Codecov Report

Merging #439 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #439   +/-   ##
=======================================
  Coverage   77.93%   77.93%           
=======================================
  Files         115      115           
  Lines       12321    12321           
=======================================
  Hits         9602     9602           
  Misses       2719     2719           

Impacted Files	Coverage Δ
Sources/MongoSwift/MongoCollection+BulkWrite.swift	91.97% <ø> (ø)
...wiftSyncTests/MongoCollection+BulkWriteTests.swift	97.48% <100.00%> (ø)

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