dev/core#1775 Fix fee amount reversals when doing a change of financi…

[seamuslee001]

…al type

Overview

This fixes an issue where by if you have a fee amount recorded for a contribution and then you do a change financial type for that contribution, the fee amount is not properly reversed in the database so the net_amount in the financial transaction is off

Before

Fee amount not properly reversed

After

Fee amount correctly reversed when doing a change of financial type

ping @eileenmcnaughton @jitendrapurohit @monishdeb @JoeMurray

[civibot]

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[eileenmcnaughton]

This seems sensible to me - I'm not a fan of the test function _checkFinancialTrxn it feels like a hack but it's non-blocking