Fix flaw in pagination for CSV file serialization

[TheKeymaster]

Purpose

There is a flaw in pagination for CSV file pagination. The file is always cleared after calling serializeItemsToFile, this makes the pagination completely useless.

Fun fact:

Calling the pagination with start 1 caused the CSV to be completely invalid, as the header is only written if the count is > 0 :laughing:

Approach

In \FINDOLOGIC\Export\CSV\CSVExporter::serializeItemsToFile when calling file_put_contents add the flag FILE_APPEND. PHP documentation:

If file filename already exists, append the data to the file instead of overwriting it.

Open Questions and Pre-Merge TODOs

• [x] composer lint and composer fix was executed.
• [x] Tests were written and pass with 100% coverage.
• [ ] A issue with a detailed explanation of the problem/enhancement was created and linked.

[codecov-io]

Codecov Report

Merging #123 into develop will not change coverage by %. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             develop      #123   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       269       270    +1     
===========================================
  Files             49        49           
  Lines            700       700           
===========================================
  Hits             700       700           

Flag	Coverage Δ	Complexity Δ
#unittests	100.00% <100.00%> (ø)	270.00 <0.00> (+1.00)

Impacted Files	Coverage Δ	Complexity Δ
src/FINDOLOGIC/Export/CSV/CSVExporter.php	100.00% <100.00%> (ø)	10.00 <0.00> (+1.00)

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