Crm 15622

CRM-15622: Fix for incorrect Fee Amount.
    
    There's a bunch of fee_amount fields in this section of the code leading
    to a lot of confusion (see CRM-15372). Many of the payment processors set
    the fee_amount field in the result array they return which sets up the
    contribution.fee_amount field, but the Authorize.Net AIM API doesn't
    return fee information, so Authorize.Net doesn't set this field. Instead
    of setting this field to 0 there, I thought it would be better to not
    require that the payment processor set this field. Since some of the
    payment processors return the same array that was passed into them and the
    payment processors don't need the fee_amount field as an argument, I think
    that setting that to NULL before passing it in gets us the correct output.
    
    Note that we still need the fix from CRM-15372 to get the
    participant.fee_amount field set correctly because the code that sets up
    the pariticpant shares the field array that the contribution creation
    uses even though they don't have the same 'fee_amount' field.