Fix display of select multiple HTML fields. Closes #3290.

rest_framework/templates/rest_framework/horizontal/select_multiple.html

@@ -16,7 +16,7 @@
         {% elif select.end_option_group %}
           </optgroup>
         {% else %}
-          <option value="{{ select.value }}" {% if select.value == field.value %}selected{% endif %}>{{ select.display_text }}</option>
+          <option value="{{ select.value }}" {% if select.value in field.value %}selected{% endif %}>{{ select.display_text }}</option>
         {% endif %}
       {% empty %}
           <option>{{ no_items }}</option>

rest_framework/templates/rest_framework/inline/select_multiple.html

@@ -15,7 +15,7 @@
           {% elif select.end_option_group %}
             </optgroup>
           {% else %}
-            <option value="{{ select.value }}" {% if select.value == field.value %}selected{% endif %}>{{ select.display_text }}</option>
+            <option value="{{ select.value }}" {% if select.value in field.value %}selected{% endif %}>{{ select.display_text }}</option>
           {% endif %}
       {% empty %}
       <option>{{ no_items }}</option>

rest_framework/templates/rest_framework/vertical/select_multiple.html

@@ -15,7 +15,7 @@
         {% elif select.end_option_group %}
           </optgroup>
         {% else %}
-          <option value="{{ select.value }}" {% if select.value == field.value %}selected{% endif %}>{{ select.display_text }}</option>
+          <option value="{{ select.value }}" {% if select.value in field.value %}selected{% endif %}>{{ select.display_text }}</option>
         {% endif %}
     {% empty %}
         <option>{{ no_items }}</option>