[CoreGraphics] Fix issue with the MakeMutable in CGPath where ref count was leaking. (#3228)

src/CoreGraphics/CGPath.cs

@@ -560,7 +560,7 @@ namespace XamCore.CoreGraphics {
 		static CGPath MakeMutable (IntPtr source)
 		{
 			var mutable = CGPathCreateMutableCopy (source);
-			return new CGPath (mutable, false);
+			return new CGPath (mutable, owns: true);
 		}
 
 		[DllImport (Constants.CoreGraphicsLibrary)]

tests/monotouch-test/CoreGraphics/PathTest.cs

@@ -275,8 +275,9 @@ namespace MonoTouchFixtures.CoreGraphics {
 				using (var copy = p1.Copy ())
 				{
 					var newRetainCount = CFGetRetainCount (copy.Handle);
-					Assert.AreNotEqual (count, newRetainCount, $"Count is {newRetainCount} and was {count}");
-					Assert.IsTrue (count < newRetainCount);
+					Assert.AreEqual (count, newRetainCount, "Ref count should not have changed.");
+					Assert.AreEqual (1, count, "Original count.");
+					Assert.AreEqual (1, newRetainCount, "New count");
 				}
 			}
 		}