Access "response" of CoreAdmin queries

[thomascorthals]

CoreAdmin queries can have a field named "response" in the JSON response data. Parsing that data assigned its value (of type array) to Solarium\Core\Query\Result\Result::$response (of type Solarium\Core\Client\Response).

This caused a TypeError when calling Result::getResponse() to get the Response object.

TypeError: Solarium\Core\Query\Result\Result::getResponse(): Return value must be of type Solarium\Core\Client\Response, array returned

It also caused an Error when trying to access Result::$response to get the value.

Error: Cannot access protected property Solarium\Tests\QueryType\Server\CoreAdmin\Result\Result::$response

This PR fixes both problems by storing it under a different property name but making it accessible under its original name through property overloading.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.16%. Comparing base (8bc45a0) to head (6fcf1e9).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1167      +/-   ##
==========================================
+ Coverage   98.13%   98.16%   +0.02%     
==========================================
  Files         404      404              
  Lines       10626    10632       +6     
==========================================
+ Hits        10428    10437       +9     
+ Misses        198      195       -3     

Flag	Coverage Δ
unittests	98.16% <100.00%> (+0.02%)	⬆️

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