Fixed expand empty not expanding tags with attributes

[M-Valts]

In PR #620 the expand_empty_elements function was added to the serializer. With the current implementation only unit variants are expanded, but if a tag has an attribute it would not expand the tag which seems inconsistent.

[Mingun]

Could you add tests for that? There are no writer-only tests so I suggest to create tests/writer-config.rs and add test for expand option there (it then will be filled with tests for other options as well, similarly to reader-config.rs).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (de61fa1) 65.06% compared to head (7b9653b) 65.31%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #704      +/-   ##
==========================================
+ Coverage   65.06%   65.31%   +0.25%     
==========================================
  Files          38       38              
  Lines       17954    18131     +177     
==========================================
+ Hits        11682    11843     +161     
- Misses       6272     6288      +16     

Flag	Coverage Δ
unittests	65.31% <100.00%> (+0.25%)	⬆️

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[Mingun]

Thanks @M-Valts. I still wait tests to ensure that now it works in all situations.

[Mingun]

Thanks, @M-Valts !