Add magic method __isset() to FeedsConfigurable to use default entity wrapping

Problem
Checking with isset() a property of an object that extends FeedsConfigurable always fails.

Description
I need this because we have FeedsSource object wrapped by default entity wrapper. During entity unwrapping we need to know whether our parameter (in my case feeds source id) is set or not. See #1593902: How to specify that a rule is for a certain feed import? for more details.

Reason
isset calling always returns FALSE because we don't have magic method __isset() implemented.

Solution
Just implement it.

I will provide a patch soon.