Added test case for a borrowed str

[VictorKoenders]

In the code there was a test struct with a &'a str field, but during the derive implementation we must've forgotten to re-enable it

[codecov-commenter]

Codecov Report

Merging #441 (e26bd3a) into trunk (f4b4a5d) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            trunk     #441      +/-   ##
==========================================
+ Coverage   61.74%   61.80%   +0.05%     
==========================================
  Files          46       46              
  Lines        3338     3343       +5     
==========================================
+ Hits         2061     2066       +5     
  Misses       1277     1277              

Impacted Files	Coverage Δ
tests/derive.rs	100.00% <100.00%> (ø)

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