title: Swift class field of type var Array is converted to immutable NSArray instead of NSMutableArray
body: <p>A simple Swift class has a field of type var Array. When the class is adapted to Objective-C the field type is exposed as NSArray (immutable) while it should be NSMutableArray (mutable)</p>
  
  <pre><code>class Categoryy: NSObject {
  
      var items = Array&lt;Item&gt;()
  
  }</code></pre>
  
  <p>The Categoryy swift class is adapted to Objective-C in the Xcode-generated header MODULE_NAME-swift.h file as following:</p>
  
  <pre><code>SWIFT_CLASS("_TtC8waiterio9Categoryy")
  @interface Categoryy : NSObject
  @property (nonatomic, copy) NSArray * items;
  - (instancetype)init OBJC_DESIGNATED_INITIALIZER;
  @end</code></pre>
  
  <p>is there a way for the var items : Array field to be converted to a mutable NSMutableArray in Objective-c?</p>