correct selection logic for QuerySetSelectMultipleField

Previously, it was using the default behavior from QuerySetSelectField.
However, this compared the current object to the list of selected values,
which meant none of the them were marked as selected in the admin.

flask_mongoengine/wtf/fields.py

@@ -85,6 +85,18 @@ def  __init__(self, label=u'', validators=None, queryset=None, label_attr='',
                   allow_blank=False, blank_text=u'---', **kwargs):
         super(QuerySetSelectMultipleField, self).__init__(label, validators, queryset, label_attr, allow_blank, blank_text, **kwargs)
 
+    def iter_choices(self):
+        if self.allow_blank:
+            yield (u'__None', self.blank_text, self.data is None)
+
+        if self.queryset == None:
+            return
+
+        self.queryset.rewind()
+        for obj in self.queryset:
+            label = self.label_attr and getattr(obj, self.label_attr) or obj
+            yield (obj.id, label, obj in self.data)
+
     def process_formdata(self, valuelist):
         if valuelist:
             if valuelist[0] == '__None':