Fix: Completing an existing contribution using completetransaction does not respect currency on financial_trxn record

The completetransaction pathway is used by multiple paths to update contributions when a payment comes in. For JIRA purposes I recreated via the api in a test that does the following

1) create a pending contribution with currency = EURO

2) call completetransaction to add an incoming payment

3) before patch the currency on the financial_trxn record is 'USD' rather than EUR

The transaction records are more visible these days so this is picked up now