Just wrapping the client collection to copy options

Using the `MongoDB\Client::selectCollection` will make the library to copy driver options, such as typeMap. We also do not need to cache the list of collections.

src/Client/Connector.php

@@ -40,7 +40,7 @@ public function __construct(\MongoDB\Client $connection) {
 	/**
 	 * Retrieves a connection.
 	 *
-	 * @return \MongoClient
+	 * @return \MongoDB\Client
 	 */
 	public function getConnection() {
 		return $this->connection;
@@ -59,17 +59,10 @@ public function setDb($name = 'test') {
 	 * Retrieves a collection object by name.
 	 *
 	 * @param string $name
-	 * @return \MongoCollection
+	 * @return \MongoDB\Collection
 	 */
 	public function collection($name) {
-		if (empty($this->collections[$name])) {
-			$this->collections[$name] = new \MongoDB\Collection(
-				$this->connection->getManager(),
-				$this->connection->selectDatabase($this->dbName),
-				$name
-			);
-		}
-		return $this->collections[$name];
+		return $this->connection->selectCollection($this->dbName, $name);
 	}
 
 }