Fixed "Credit Memo not created when refund issued by merchant"

[luigifab]

Description

Sometimes, the total of the order and the total of the refund is a little different, +/- 0.001, so when an admin create a refund from PayPal website (or another payment method), we have:

IPN "Refunded". Refund issued by merchant. Registered notification about refunded amount of -61.79 €. 
Transaction ID: "xxxx". Credit Memo has not been created. Please create offline Credit Memo.

For this example the order total is 61.79:

This PR require php-bcmath. perhaps we can replace bccomp with another php code to not require on bcmath.

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