Fix invalid serialization for lists of unions

[lovasoa]

Fixes #32

[codecov]

Codecov Report

Merging #33 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #33   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           67        69    +2     
  Branches         5         5           
=========================================
+ Hits            67        69    +2     

Impacted Files	Coverage Δ
src/marshmallow_union/__init__.py	100.00% <100.00%> (ø)
tests/test_union.py	100.00% <100.00%> (ø)

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[rllin]

👍 to merging this fix!