Fix fee value in the Fiscalizer::Serializer::Invoice

[viktorzimmermann]

When fiscalising Invoice with Fee
Fiscalizer::Fee.new(name: 'Povratna naknada', value: 0.5)

Response from Porezna is:

Poruka nije u skladu s XML shemom : cvc-simple-type 1: element {http://www.apis-it.hr/fin/2012/types/f73}IznosN value '0.5' is not a valid instance of type {http://www.apis-it.hr/fin/2012/types/f73}IznosType

🙄

I noticed that there is already a methods value_str in the Fiscalizer::Fee so we can just use that in the serializer?

    def value_str
      format('%15.2f', value).strip
    end