fix KeyValueBasedTransformer

[steffyP]

This PR fixes a problem with KeyValueBasedTransformer: If replace_reference was set to False, it would replace the entire key.

Fix:

• now it replaces the returned value only (which can be a subset of the value), with the replacement string
• behaves similar to replace_reference=True

[github-actions[bot]]

LocalStack integration with Pro

       3 files         3 suites   1h 2m 54s :stopwatch: 1 177 tests 1 136 :heavy_check_mark: 41 :zzz: 0 :x: 1 542 runs  1 470 :heavy_check_mark: 72 :zzz: 0 :x:

Results for commit e65d92ff.

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