file-exporter: Use strict encoding when writing files.

[winder]

Summary

Use strict encoding when writing the block files.

Test Plan

New unit tests.

[codecov]

Codecov Report

Merging #1488 (bfe704c) into develop (c462ef0) will decrease coverage by 0.02%.
The diff coverage is 91.66%.

@@             Coverage Diff             @@
##           develop    #1488      +/-   ##
===========================================
- Coverage    65.44%   65.42%   -0.02%     
===========================================
  Files           82       82              
  Lines        11447    11459      +12     
===========================================
+ Hits          7491     7497       +6     
- Misses        3387     3391       +4     
- Partials       569      571       +2     

Impacted Files	Coverage Δ
util/util.go	84.15% <90.47%> (+1.11%)	⬆️
...duit/plugins/exporters/filewriter/file_exporter.go	76.78% <100.00%> (ø)
conduit/plugins/importers/filereader/filereader.go	88.33% <100.00%> (ø)
fetcher/fetcher.go	62.71% <0.00%> (-1.28%)	⬇️
idb/postgres/postgres.go	65.49% <0.00%> (-0.14%)	⬇️

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