Improve product -> master attributes delegation

[kennyadsl]

With #2427 we found out that we are delegating some methods to master variant incorrectly. Some of the attributes defined in MASTER_ATTRIBUTES only have their setter or getter corresponding method on the variant side.

I made this little script to verify which one would trigger a NoMethodError:

product = Spree::Product.last

Spree::Product::MASTER_ATTRIBUTES.each do |attribute|
  begin  
    product.send(attribute) 
  rescue Exception => e  
    puts "#{attribute} has no getter" if e.is_a? NoMethodError 
  end
  
  begin
    product.send("#{attribute}=", 'test') 
  rescue Exception => e  
    puts "#{attribute} has no setter" if e.is_a? NoMethodError 
  end
end

Running this in console produces this output:

currency has no getter
currency has no setter
rebuild_vat_prices has no getter
price_in has no setter
price_for has no setter
amount_in has no setter

This leads me in changing how we are defining those delegations.

[mamhoff]

Thank you, much more clear. Sweet!