Add regression tests for fixed issues

[Mingun]

A set of tests copied directly from issues (with minimal changes) just to be sure, that reported problems solved. Although each problem already has its own set of tests testing it, these tests make sure that we do not have any hidden interaction effects.

Included tests for:

• Newlines in attributes are lost #115
• Help serializing empty attributes #252
• Serialization of Hashmap duplicate outer tag in the inners elements. Deserialization has no such problem and fails on serialized xml. #343
• Error when Deserializing Nested Enum Fields #349
• Send events through channel #360
• Serializing $value enum variant doesn't work #429
• Sequences of enums at root level are not parsed correctly #500

[codecov-commenter]

Codecov Report

Merging #542 (788982b) into master (662fc24) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #542   +/-   ##
=======================================
  Coverage   61.31%   61.31%           
=======================================
  Files          32       32           
  Lines       15639    15639           
=======================================
  Hits         9589     9589           
  Misses       6050     6050           

Flag	Coverage Δ
unittests	61.31% <ø> (ø)

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