Fix index wide finder config check

[merk]

Turns out isset() doesnt work as expected for null values. Not sure how I missed this in my initial testing. :S

Fixes #61

[jmikola]

Using array_key_exists() means it would be impossible to override the finder value in a configuration (consider multiple config blocks being merged). What about:

    /**
     * Adds the configuration for the "indexes" key
     */
    private function addIndexesSection(ArrayNodeDefinition $rootNode)
    {
        $rootNode
            ->fixXmlConfig('index')
            ->children()
                ->arrayNode('indexes')
                    ->useAttributeAsKey('name')
                    ->prototype('array')
                        ->performNoDeepMerging()
                        ->children()
                            ->scalarNode('client')->end()
                            ->scalarNode('finder')
                                ->treatNullLike(true)
                                ->defaultFalse()
                            ->end()

Using YAML as an example, that would allow users to enable a finder with true and ~, but disable it with false. And in the extension, we could replace the isset() with a simple variable check since the key should always exist.

[merk]

Cant say I knew about treatNullLike. Thanks. Changed.

[jmikola]

Thanks!