set distinct.query as document

[bazile-clyde]

This commit fixes the issue from the task test-latest-server-auth-sasl-openssl-cse on GCC 8.3 here. Unfortunately, that task now has another unrelated error.

Here's a patch showing the previously failing tasks succeeding (link). I also verified this locally.

[bazile-clyde]

From MongoDB's docs here, the query field of a distinct command should be a document. Previously, we were setting it as a string.

See the docs for BCON_NEW for an example of how to use this function.

[codecov-io]

Codecov Report

Merging #748 (354abff) into master (4184f37) will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #748      +/-   ##
==========================================
- Coverage   60.91%   60.80%   -0.11%     
==========================================
  Files         278      278              
  Lines       73398    73398              
==========================================
- Hits        44709    44630      -79     
- Misses      28689    28768      +79     

Impacted Files	Coverage Δ
src/libmongoc/tests/unified/operation.c	23.51% <ø> (ø)
src/libmongoc/tests/test-mongoc-async.c	3.81% <0.00%> (-25.20%)	⬇️
src/libmongoc/tests/test-mongoc-primary-stepdown.c	3.04% <0.00%> (-9.14%)	⬇️
src/libmongoc/tests/test-mongoc-counters.c	66.56% <0.00%> (-6.39%)	⬇️
src/libmongoc/src/mongoc/mongoc-read-prefs.c	85.61% <0.00%> (-0.72%)	⬇️
src/libmongoc/tests/test-mongoc-stream-tls-error.c	96.95% <0.00%> (-0.61%)	⬇️
src/libbson/src/bson/bson-iter.c	89.71% <0.00%> (-0.54%)	⬇️
src/libmongoc/tests/test-mongoc-socket.c	98.28% <0.00%> (-0.43%)	⬇️
src/libmongoc/tests/unified/runner.c	66.33% <0.00%> (-0.15%)	⬇️
src/libmongoc/src/mongoc/mongoc-server-monitor.c	62.04% <0.00%> (+0.20%)	⬆️

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