Setter for changing the query's keys.

Issue Summary

It would be great if other modules could change the search keys in the current search query.

For example, in one of our sites. We need to search on a given field, when a searchmode is given.

We do the following in the apachesolr_modify_query hook:
if (isset($_GET['searchmode'])) {
$params['qt'] = 'standard';
$query->setKeys($_GET['searchmode'] .':'. search_get_keys());
}

Following was added in Solr_Base_Query.php
/**
* Set the search keywords
*/
public function setKeys($keys) {
$this->keys = $keys;
}