Some tests demonstrating copying behavior of immutable and mutable dictionaries.

DictionaryCopyingTests.m

- (void)testCopyReturnsImmutable
{
    NSDictionary* immutable = @{@"foo": @"bar"};
    NSMutableDictionary* mutable = [immutable mutableCopy];
    NSDictionary* copyOfMutable = [mutable copy];
    XCTAssertFalse([copyOfMutable isMemberOfClass:[NSMutableDictionary class]]);
    // interestingly, the class is actually __NSDictionaryI, so you can't assert isMemberOfClass:NSDictionary
    XCTAssertTrue([copyOfMutable isKindOfClass:[NSDictionary class]]);
}

- (void)testCopyOfImmutableReturnsSelf
{
    NSDictionary* test = @{@"foo": @1};
    XCTAssertEqual(test, [test copy], @"Expected copies of immutable objects to be the objects themselves.");
}