Only numbers should be num-coercable

[gphilipp]

If you have a union type like ["long" "string"], the avro serde would fail to serialise a string which contains a numerical value, eg: "123". I added a test case for that. The fix simply checks if the passed value to the num-coercable? function is actually a number.

[codecov]

Codecov Report

Merging #203 into master will increase coverage by 1.27%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #203      +/-   ##
=========================================
+ Coverage   79.52%   80.8%   +1.27%     
=========================================
  Files          41      39       -2     
  Lines        2530    2365     -165     
  Branches      151     149       -2     
=========================================
- Hits         2012    1911     -101     
+ Misses        367     305      -62     
+ Partials      151     149       -2

Impacted Files	Coverage Δ
src/jackdaw/serdes/avro.clj	87.92% <100%> (+0.03%)	⬆️
src/jackdaw/serdes/edn.clj	80.76% <0%> (-19.24%)	⬇️
src/jackdaw/test/commands.clj	86.95% <0%> (-13.05%)	⬇️
src/jackdaw/test/fixtures.clj	55.7% <0%> (-11.18%)	⬇️
src/jackdaw/admin.clj	83.48% <0%> (-5.51%)	⬇️
src/jackdaw/streams/interop.clj	78.68% <0%> (-1.6%)	⬇️
src/jackdaw/client.clj	78.51% <0%> (-1.22%)	⬇️
src/jackdaw/test.clj	77.77% <0%> (-1.02%)	⬇️
src/jackdaw/streams.clj	81.96% <0%> (-0.3%)	⬇️
src/jackdaw/test/serde.clj	94.44% <0%> (ø)	⬆️
... and 10 more

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