title: Using addObjectsFromArray on a nil NSMutableSet
body: <p>I have an array (arrayA) of type NSArray having custom objects in it and a NSMutableSet (setA) also having the custom objects of same class. I wish to merge the contents in both of these in a set to avoid duplicate values, so I used <code>[setA addObjectsFromArray:arrayA]</code> and then I print setA.</p>
  
  <p>I have a scenario where in setA would be nil and arrayA would have some contents in it. My understanding was even if setA is nil, <code>[setA addObjectsFromArray:arrayA]</code> would add contents of arrayA in SetA, but when I print setA I get nil instead of objects in setA.</p>
  
  <p>How does this "addObjectsFromArray" work exactly? or am I missing something here? Apple docs didn't give me much idea about the nil part of the set.</p>