def body(klass = Builder)
      builder = klass.new(body_content, input_body_content_type)
      yield builder if block_given?
      builder
    end
def body(klass = Builder)
      builder = klass.new(body_content, input_body_content_type)
      yield builder if block_given?
      builder
    end
Build the body of the envelope

 @example
   env.body do |b|
     b.some 'data'
   end