couchdb: Fixes query for document existance

The originaly implementation did not properly take into account the
promise returned from the maybe function querying the document.  This
resulted in a promise being compared to null which will false.

Fixes the logic via a cast to boolean on the document result.

couchdb.js

@@ -137,8 +137,8 @@ class Database {
 	 * @return true if the document exists, otherwise false
 	 */
 	async exists( id ){
-		const document = this.maybe_by_id(id);
-		return null == document;
+		const document = await this.maybe_by_id(id);
+		return !!document;
 	}
 }