Edit JSONPlugin class so that list objects are automatically encoded as JSON

Hi Marcel, I've been using bottle for about a year now and love it, but wanted to suggest this change, as it's an issue I've run into several times:

Having a list as the root element of a JSON document is perfectly valid. The auto-decoding of JSON documents with lists at the root works just fine; this fix allows bottle to also automatically encode list objects to JSON.

(Also, this is my first time using GitHub's Pull Request feature, and I'm relatively new to Git also, so if I'm doing something wrong, please be gentle!)