Restore support for "accept single value as array" in config?

[sacsar]

In looking at the diff in #39, I noticed that DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY was set to true, but, after #31 was merged, this effectively became false. This behavior wasn't documented or tested, but could be restored if desired. (I assume it was intended to be used with the labels field.)