Test if read_only is defined on the form view

This option does not exist anymore on Symfony 3.0+

Resources/views/Form/Type/sonata_type_model_autocomplete.html.twig

@@ -11,7 +11,7 @@ file that was distributed with this source code.
 {% spaceless %}
 
     <input type="text" id="{{ id }}_autocomplete_input" value=""
-        {%- if read_only %} readonly="readonly"{% endif -%}
+        {%- if read_only is defined and read_only %} readonly="readonly"{% endif -%}
         {%- if disabled %} disabled="disabled"{% endif -%}
         {%- if required %} required="required"{% endif %}
     />
@@ -34,7 +34,7 @@ file that was distributed with this source code.
                 placeholder: '{{ placeholder ?: allowClearPlaceholder }}', // allowClear needs placeholder to work properly
                 allowClear: {{ required ? 'false' : 'true' }},
                 enable: {{ disabled ? 'false' : 'true' }},
-                readonly: {{ read_only ? 'true' : 'false' }},
+                readonly: {{ read_only is defined and read_only or attr.readonly is defined and attr.readonly ? 'true' : 'false' }},
                 minimumInputLength: {{ minimum_input_length }},
                 multiple: {{ multiple ? 'true' : 'false' }},
                 width: '{{ width }}',