Change the way of assigning template variables

`ActionView::Base#assign` method removes all previous view variable assignments from ActionView::Base `assigns` hash if there are any.

For example:

    template.assigns # => {page_title: "Awesome Title"}
    template.assign(has_many_block: true) # this will clear the `assigns` hash
    template.assigns # => {has_many_block: true}

This commit changes the way of assigning template variables, to avoid previously assigned values loss:

    template.assigns[:has_many_block] = true

lib/active_admin/form_builder.rb

@@ -62,7 +62,7 @@ def has_many(assoc, options = {}, &block)
           template.concat has_many_actions(has_many_form, builder_options, "".html_safe)
         end
 
-        template.assign(has_many_block: true)
+        template.assigns[:has_many_block] = true
         contents = without_wrapper { inputs(options, &form_block) } || "".html_safe
 
         if builder_options[:new_record]

lib/active_admin/views/components/active_admin_form.rb

@@ -43,7 +43,7 @@ def build(resource, options = {}, &block)
 
       def inputs(*args, &block)
         if block_given?
-          form_builder.template.assign(has_many_block: true)
+          form_builder.template.assigns[:has_many_block] = true
         end
         if block_given? && block.arity == 0
           wrapped_block = proc do