Fix mutating array datasource bug

Summary:
Copy objects when retrieving from datasource to account for the edge case where the returned data is a mutable array.

Issue fixed: #999

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/1109

Differential Revision: D7246645

Pulled By: rnystrom

fbshipit-source-id: d9b0c2ba07983bf46327d4ee1ba0eba2e194ba19

CHANGELOG.md

@@ -12,6 +12,7 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 - 5x improvement to diffing performance when result is only inserts or deletes. [Ryan Nystrom](https://github.com/rnystrom) [(tbd)](tbd)
 
 ### Fixes
+- Copy objects when retrieving from datasource to prevent modification of models in binding section controller. [Kashish Goel](https://github.com/kashishgoel) [(#1109)](https://github.com/Instagram/IGListKit/pull/1109)
 
 3.2.0
 -----

Source/IGListBindingSectionController.m

@@ -114,7 +114,7 @@ typedef NS_ENUM(NSInteger, IGListDiffingSectionState) {
     self.object = object;
 
     if (oldObject == nil) {
-        self.viewModels = [self.dataSource sectionController:self viewModelsForObject:object];
+        self.viewModels = [[self.dataSource sectionController:self viewModelsForObject:object] copy];
     } else {
         IGAssert([oldObject isEqualToDiffableObject:object],
                  @"Unequal objects %@ and %@ will cause IGListBindingSectionController to reload the entire section",

Tests/IGListBindingSectionControllerTests.m

@@ -296,5 +296,24 @@
     [self waitForExpectationsWithTimeout:30 handler:nil];
 }
 
+- (void)test_whenUpdating_withMutableArrayObject_thatViewModelsDontMutate {
+    NSArray *objects = @[
+                             @"foo",
+                             @"bar"
+                             ];
+    NSMutableArray *initObjects = [NSMutableArray arrayWithArray:objects];
+    
+    [self setupWithObjects:@[
+                             [[IGTestDiffingObject alloc] initWithKey:@1 objects:initObjects]
+                             ]];
+    
+    IGTestDiffingSectionController *section = [self.adapter sectionControllerForObject:self.dataSource.objects.firstObject];
+    
+    NSArray *oldModels = [section.viewModels copy];
+    [initObjects removeAllObjects];
+    
+    XCTAssertEqual(oldModels, section.viewModels);
+}
+
 @end