Allow backend productAutocomplete to support a custom ransack query

From this change, when initializing a productAutocomplete, a callback
can be given to the ransack_q_callback option. This callback should
return a hash for the ransack search. The callback also allows for one
argument, which is the string typed in the select2 (term).

backend/app/assets/javascripts/spree/backend/product_picker.js

@@ -4,6 +4,15 @@ $.fn.productAutocomplete = function (options) {
  // Default options
  options = options || {}
  var multiple = typeof(options['multiple']) !== 'undefined' ? options['multiple'] : true
+ function ransackQuery(term) {
+ if (typeof(options['ransack_q_callback']) === 'function') return options['ransack_q_callback'](term)
+
+ return {
+ name_cont: term,
+ variants_including_master_sku_start: term,
+ m: 'or'
+ }
+ }
 
  function formatProduct(product) {
  return Select2.util.escapeMarkup(product.name);
@@ -27,11 +36,7 @@ $.fn.productAutocomplete = function (options) {
  params: { "headers": { 'Authorization': 'Bearer ' + Spree.api_key } },
  data: function (term, page) {
  return {
- q: {
- name_cont: term,
- variants_including_master_sku_start: term,
- m: 'or'
- },
+ q: ransackQuery(term),
  token: Spree.api_key,
  page: page
  };