[DDC-3170] SimpleObjectHydrator fails to get discriminator column from mapped SQL result

This PR fixes [DDC-3170](http://www.doctrine-project.org/jira/browse/DDC-3170).

When querying a simple entity which uses single table- or class table inheritance using simple object hydration (``AbstractQuery::HYDRATE_SIMPLEOBJECT``), the mapped discriminator column was not retrieved correctly.

If the column got an alias during result set mapping other than it's actual name (e.g. ``type34`` instead of ``type``) than this alias wasn't correctly resolved when retrieving the discriminator column from the SQL result set.
@Ocramius Thanks for the feedback. I feared that this PR was just swallowed into the master branch ;)

Please note though, ``array_search`` is _quite_ fast:
```php
$t = microtime(true);
$a = array_fill(0, 10, 'foo');
for($i=0; $i < 1000; $i++) array_search('foobar', $a);
echo (microtime(true) - $t)*1000;
// -> ~0.75ms on a normal Core i5
```
This example used a ``metaMappings`` array with ten records. As only foreign keys and discriminator columns are put in the metaMappings array it usually should have fewer then five records in which case you could hydrate 2 000 in ~1ms.

And hydrating 2 000 records can take up more than 100ms, even when using the ``SimpleObjectHydrator``. I don't think that 1ms more would really matter.

I could cache the result though. What do you think of this:
```php
if (isset($cache['_dcn'])) {
  $discrColumnName = $cache['_dcn'];
} else {
  $discrColumnName = $this->_platform->getSQLResultCasing($this->class->discriminatorColumn['name']);

  // Find mapped discriminator column from the result set.
  if ($metaMappingDiscrColumnName = array_search($discrColumnName, $this->_rsm->metaMappings)) {
    $discrColumnName = $metaMappingDiscrColumnName;
  }
}
```

As column aliases for the ``SimpleObjectHydrator`` have numbered suffixes, there won't be a column using the alias ``_dcn`` so there won't be any cache collisions. The key is rather small so processing time will be smaller too.

I just ran the tests and had no problems. If it's o.k. with you, I would change the PR accordingly.