oops: Fixup plugin configuration loading

The code introduced to convert the database values to PHP values introduced
a bug where the configuration would not be saved.

include/class.plugin.php

@@ -9,8 +9,12 @@ class PluginConfig extends Config {
         // Use parent constructor to place configurable information into the
         // central config table in a namespace of "plugin.<id>"
         parent::Config("plugin.$name");
-        foreach ($this->getOptions() as $name => $field)
-            $this->config[$name]['value'] = $field->to_php($this->get($name));
+        foreach ($this->getOptions() as $name => $field) {
+            if ($this->exists($name))
+                $this->config[$name]['value'] = $field->to_php($this->get($name));
+            elseif ($default = $field->get('default'))
+                $this->defaults[$name] = $default;
+        }
     }
 
     /* abstract */