fix bug in changing custom field option

bug description :

> if you change the options of a custom field choice (i.e. from a
> multiple to a single, removing or adding allow_other), the data
> representation change and do not match with the expected
> representation of the form.

This commit fix this bug by switching the data representation to the
current options.

CustomFields/CustomFieldChoice.php

@@ -167,8 +167,96 @@ class CustomFieldChoice extends AbstractCustomField
 
     public function deserialize($serialized, CustomField $customField)
     {
+        // we always have to adapt to what the current data should be
+        $options = $customField->getOptions();
+        
+        if ($options[self::MULTIPLE]) {
+            return $this->deserializeToMultiple($serialized, $options[self::ALLOW_OTHER]);
+        } else {
+            return $this->deserializeToUnique($serialized, $options[self::ALLOW_OTHER]);
+        }
         return $serialized;
     }
+    
+    private function deserializeToUnique($serialized, $allowOther)
+    {
+        $value = $this->guessValue($serialized);
+        
+        // set in a single value. We must have a single string
+        $fixedValue = is_array($value) ? 
+                // check if the array has an element, if not replace by empty string
+                count($value) > 0 ? end($value) : '' 
+                : 
+                $value;
+        
+        if ($allowOther) {
+            return $this->deserializeWithAllowOther($serialized, $fixedValue);
+        } else {
+            return $fixedValue;
+        }
+    }
+    
+    /**
+     * deserialized the data from the database to a multiple 
+     * field
+     * 
+     * @param mixed $serialized
+     * @param boolean $allowOther
+     */
+    private function deserializeToMultiple($serialized, $allowOther)
+    {
+        $value = $this->guessValue($serialized);
+        
+        // set in an array : we want a multiple
+        $fixedValue = is_array($value) ? $value : array($value);
+        
+        if ($allowOther) {
+            return $this->deserializeWithAllowOther($serialized, $fixedValue);
+        } else {
+            return $fixedValue;
+        }
+    }
+    
+    private function deserializeWithAllowOther($serialized, $value)
+    {
+        $existingOther = isset($serialized['_other']) ? $serialized['_other'] : '';
+        
+        return array(
+            '_other' => $existingOther,
+            '_choices' => $value
+        );
+    }
+    
+    /**
+     * Guess the value from the representation of it.
+     * 
+     * If the value had an 'allow_other' = true option, the returned value
+     * **is not** the content of the _other field, but the `_other` string.
+     * 
+     * @param array|string $value
+     * @return mixed
+     * @throws \LogicException if the case is not covered by this
+     */
+    private function guessValue($value)
+    {
+        if ($value === NULL) {
+            return NULL;
+        }
+        
+        if (!is_array($value)) {
+            return $value;
+        } else {
+            // we have a field with "allow other"
+            if (isset($value['_choices'])) {
+                return $value['_choices'];
+            } else {
+                // we have a field with "multiple"
+                return $value;
+            }
+        }
+        
+        throw \LogicException("This case is not expected.");
+    }
 
     public function getName()
     {