Issue-3465570 Fix CSV header generation

Renuka Kulkarnirequested to merge
issue/contentimport-3465570:3465570-ajax-500-error into 10.0.x

Closes #3465570 Updated the sample CSV generator to prepend commas when appending field names and replaced substr() with rtrim() to prevent merged columns such as langcode and promote appearing in a single column.

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