Support multiple="multiple" attribute.

Some html preprocessors will transform

<ui-select multiple></ui-select>

into

<ui-select multiple="multiple"></ui-select>

which is currently not supported by angular-ui-select. This patch fixes the
issue.

Author: ctavan

src/select.js

@@ -632,6 +632,7 @@
 
         $select.multiple = angular.isDefined(attrs.multiple) && (
             attrs.multiple === '' ||
+            attrs.multiple.toLowerCase() === 'multiple' ||
             attrs.multiple.toLowerCase() === 'true'
         );
 

test/select.spec.js

@@ -1520,6 +1520,22 @@ describe('ui-select tests', function() {
          .toBe("Wladimir <wladimir@email.com>Samantha <samantha@email.com>Nicole <nicole@email.com>");
 
     });
+
+    it('should support multiple="multiple" attribute', function() {
+
+      var el = compileTemplate(
+          '<ui-select multiple="multiple" ng-model="selection.selectedMultiple" theme="bootstrap" style="width: 800px;"> \
+              <ui-select-match placeholder="Pick one...">{{$item.name}} &lt;{{$item.email}}&gt;</ui-select-match> \
+              <ui-select-choices repeat="person.email as person in people | filter: $select.search"> \
+                <div ng-bind-html="person.name | highlight: $select.search"></div> \
+                <div ng-bind-html="person.email | highlight: $select.search"></div> \
+              </ui-select-choices> \
+          </ui-select> \
+          '
+      );
+
+      expect(el.scope().$select.multiple).toBe(true);
+    });
   });
 
   describe('default configuration via uiSelectConfig', function() {