Make type affinity detection more robust

[quinnj]

Fixes #242.

[codecov]

Codecov Report

Merging #264 (276f5ab) into master (339cb22) will decrease coverage by 0.72%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
- Coverage   85.27%   84.54%   -0.73%     
==========================================
  Files           5        5              
  Lines         686      686              
==========================================
- Hits          585      580       -5     
- Misses        101      106       +5     

Impacted Files	Coverage Δ
src/SQLite.jl	95.54% <100.00%> (-0.41%)	⬇️
src/UDF.jl	84.49% <0.00%> (-1.56%)	⬇️
src/api.jl	53.50% <0.00%> (-1.28%)	⬇️

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