MDL-36760 formslib: fix frozen elements with [] in name.

This was a regression caused by MDL-30845 or, to be precise, the related
issue MDL-32785.

Form elements with names like multipier[0] require special handling,
such as is present in HTML_QuickForm_element::_prepareValue. I have
added equivalent handling to MoodleQuickForm::exportValues. I am afraid
that I could not think of a way to do this reliably without duplicating
code.

lib/formslib.php

@@ -1670,7 +1670,7 @@ class MoodleQuickForm extends HTML_QuickForm_DHTMLRulesTableless {
                     $value = '';
                     // If we have a default value then export it.
                     if (isset($this->_defaultValues[$varname])) {
-                        $value = array($varname => $this->_defaultValues[$varname]);
+                        $value = $this->prepare_fixed_value($varname, $this->_defaultValues[$varname]);
                     }
                 } else {
                     $value = $this->_elements[$key]->exportValue($this->_submitValues, true);
@@ -1700,6 +1700,29 @@ class MoodleQuickForm extends HTML_QuickForm_DHTMLRulesTableless {
         }
         return $unfiltered;
     }
+    /**
+     * This is a bit of a hack, and it duplicates the code in
+     * HTML_QuickForm_element::_prepareValue, but I could not think of a way or
+     * reliably calling that code. (Think about date selectors, for example.)
+     * @param string $name the element name.
+     * @param mixed $value the fixed value to set.
+     * @return mixed the appropriate array to add to the $unfiltered array.
+     */
+    protected function prepare_fixed_value($name, $value) {
+        if (null === $value) {
+            return null;
+        } else {
+            if (!strpos($name, '[')) {
+                return array($name => $value);
+            } else {
+                $valueAry = array();
+                $myIndex  = "['" . str_replace(array(']', '['), array('', "']['"), $name) . "']";
+                eval("\$valueAry$myIndex = \$value;");
+                return $valueAry;
+            }
+        }
+    }
+
     /**
      * Adds a validation rule for the given field
      *