changing xml "frete_total" to "frete" according to Akatus documentation

According to the XML schema provided by Akatus ( https://github.com/Akatus/AkatusXMLSchema/blob/master/cart.xsd#L445 ) the correct node name is "frete" and not "frete_total"
updating xml fields for transactions ("desconto_total" and "peso_total")

the nodes "desconto_total" and "peso_total" are wrong according to Akatus XML Schema
Merge pull request #5 from tiagoamaro/patch-1

changing xml "frete_total" to "frete" according to Akatus documentation