CRM-14881 - avoid error when saving contribution made by authnet

We should only check the sum of the fee and net if there is a fee
and/or net that is not equal to 0.00.

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* CRM-14881: Cannot edit contribution created by authorize.net recurring contribution
  https://issues.civicrm.org/jira/browse/CRM-14881
CRM-14881 - ensure we use the default NULL rather than 0.00 for fee/net
amounts so it's consistent with other IPNs.

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* CRM-14881: Cannot edit contribution created by authorize.net recurring contribution
  https://issues.civicrm.org/jira/browse/CRM-14881
Merge pull request #3586 from jmcclelland/auth-net-amount-fee-amount-sum

Auth net amount fee amount sum