fix: handle 'desc' order in QuerySet.orderBy

[tomasz-zablocki]

closes #309

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Fix to convert function bug, incorrectly handling false and 'desc' values.

[codecov-io]

Codecov Report

Merging #311 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #311      +/-   ##
==========================================
+ Coverage   99.81%   99.81%   +<.01%     
==========================================
  Files          28       28              
  Lines        1083     1086       +3     
  Branches      219      219              
==========================================
+ Hits         1081     1084       +3     
  Misses          2        2

Impacted Files	Coverage Δ
src/QuerySet.js	100% <ø> (ø)	⬆️
src/db/Table.js	98.9% <100%> (+0.01%)	⬆️

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[tomasz-zablocki]

thx @haveyaseen

[ghost]

@tomasz-zablocki @haveyaseen Where exactly can you use orderBy? It's not working in my selectors. It says it's not a function.