[#1259] Fixes a JS error with autocomplete module and improves UX

1. Basically adds an extra check to make sure that 'abort()' is
defined
2. Then changes the module to behave a little nicer when inbetween
ajax requests

ckan/public/base/javascript/modules/autocomplete.js

@@ -86,6 +86,8 @@ this.ckan.module('autocomplete', function (jQuery, _) {
       $('.select2-choice', select2.container).on('click', function() {
         return false;
       });
+
+      this._select2 = select2;
     },
 
     /* Looks up the completions for the current search term and passes them
@@ -140,29 +142,30 @@ this.ckan.module('autocomplete', function (jQuery, _) {
       // old data.
       this._lastTerm = string;
 
+      // Kills previous timeout
+      clearTimeout(this._debounced);
+
+      // OK, wipe the dropdown before we start ajaxing the completions
+      fn({results:[]});
+
       if (string) {
-        if (!this._debounced) {
-          // Set a timer to prevent the search lookup occurring too often.
-          this._debounced = setTimeout(function () {
-            var term = module._lastTerm;
+        // Set a timer to prevent the search lookup occurring too often.
+        this._debounced = setTimeout(function () {
+          var term = module._lastTerm;
 
-            delete module._debounced;
+          // Cancel the previous request if it hasn't yet completed.
+          if (module._last && typeof module._last.abort == 'function') {
+            module._last.abort();
+          }
 
-            // Cancel the previous request if it hasn't yet completed.
-            if (module._last) {
-              module._last.abort();
-            }
+          module._last = module.getCompletions(term, function (terms) {
+            fn(terms);
+          });
+        }, this.options.interval);
 
-            module._last = module.getCompletions(term, function (terms) {
-              fn(module._lastResults = terms);
-            });
-          }, this.options.interval);
-        } else {
-          // Re-use the last set of terms.
-          fn(this._lastResults || {results: []});
-        }
-      } else {
-        fn({results: []});
+        // This forces the ajax throbber to appear, because we've called the
+        // callback already and that hides the throbber
+        $('.select2-search input', this._select2.dropdown).addClass('select2-active');
       }
     },