def headers(h = nil)
      if h
        raise ArgumentError, 'Headers must be an object which responds to #to_hash' unless h.respond_to?(:to_hash)
        default_options[:headers] ||= {}
        default_options[:headers].merge!(h.to_hash)
      else
        default_options[:headers] || {}
      end
    end
def headers(h = nil)
      if h
        raise ArgumentError, 'Headers must be an object which responds to #to_hash' unless h.respond_to?(:to_hash)
        default_options[:headers] ||= {}
        default_options[:headers].merge!(h.to_hash)
      else
        default_options[:headers] || {}
      end
    end
Allows setting HTTP headers to be used for each request.

   class Foo
     include HTTParty
     headers 'Accept' => 'text/html'
   end