def add_field(name, value)
      return if name.blank? || value.blank?
      @fields[name.to_s] = value.to_s
    end
def add_field(name, value)
      return if name.blank? || value.blank?
      @fields[name.to_s] = value.to_s
    end
Add an input in the form. Useful when gateway requires some uncommon
 fields not provided by the gem.

 # inside `payment_service_for do |service|` block
 service.add_field('c_id','_xclick')

 Gateway implementor can also use this to add default fields in the form:

 # in a subclass of ActiveMerchant::Billing::Integrations::Helper
 def initialize(order, account, options = {})
 super
 # mandatory fields
 add_field('Rvg2c', 1)
 end

 @param name [String] input name
 @param value [String] input value