when form_for is used with custom builder don't expect association_na…

…me to be passed as option

This way you can use:
  f.form_for :books, builder: Books::FormBuilder do |ff|
    ff.title

and in Books::FormBuilder you can create method #render_title (no
association in between render and title parts).
This allows you to reuse form builder to render single book for example,
or simply split form builder into many smaller form builders.

releaf-core/app/helpers/releaf/builders/form_builder.rb

@@ -13,7 +13,7 @@ def field_render_method_name(name)
 
     builder = self
     until builder.options[:parent_builder].nil? do
-      parts << builder.options[:relation_name]
+      parts << builder.options[:relation_name] if builder.options[:relation_name]
       builder = builder.options[:parent_builder]
     end