added the possibility to use column names as keys for the types in Connection::insert()

The Connection::insert() method takes an array of types as its third argument:

```php
$conn->insert('foo', array('foo' => 'foo', 'date' => new \DateTime()), array(2 => 'datetime'));
```

But without looking at the implementation, I'm sure you would have written the following instead:

```php
$conn->insert('foo', array('foo' => 'foo', 'date' => new \DateTime()), array('date' => 'datetime'));
```

But unfortunately, that's does not work. This commit add support for this, and of course, the old way still works.