fixes #934

[kouralex]

This PR fixes #934 pretty straightforward except for

1) ConceptPropertyTest::testAddValue
This test did not perform any assertions
/skosmos/tests/ConceptPropertyTest.php:79

2) ConceptTest::testGetPropertiesAlphabeticalSortingOfPropertyValues
This test did not perform any assertions
/skosmos/tests/ConceptTest.php:178

Both of which require a more detailed intervention and changes in logic. One can see #938 for example implementation.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
1 Code Smell

No Coverage information
17.6% Duplication

[osma]

Looks good! One of the Travis build jobs (PHP 7.3) failed, probably due to a transient issue. I restarted it. When it finishes, this can be merged.

[codecov]

Codecov Report

Merging #946 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #946      +/-   ##
============================================
+ Coverage     58.38%   58.45%   +0.07%     
  Complexity     1486     1486              
============================================
  Files            32       32              
  Lines          4162     4162              
============================================
+ Hits           2430     2433       +3     
+ Misses         1732     1729       -3

Impacted Files	Coverage Δ	Complexity Δ
model/Model.php	81.52% <0%> (+1.2%)	105% <0%> (ø)	⬇️

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