Properly support NSSecureCoding by listing needed classes.

While not clear from the docs, it seems NSDictionary/NSArray when using
SecureCoding will extract strings and numbers just fine, they don't like other
classes unless explicitly listed.

Fixes #298

Source/Objects/GTLRObject.m

@@ -123,8 +123,14 @@ + (BOOL)supportsSecureCoding {
 - (instancetype)initWithCoder:(NSCoder *)decoder {
   self = [super init];
   if (self) {
-    _json = [decoder decodeObjectOfClass:[NSMutableDictionary class]
-                                  forKey:kGTLRObjectJSONCoderKey];
+    // NSDictionary/NSArray seem to allow strings and numbers with secure coding
+    // just fine, but to allow sub arrays or dictionaries (or an null) the
+    // classes have to be explicitly listed to decode correctly.
+    NSSet *expectedClasses =
+        [NSSet setWithObjects:
+           [NSMutableDictionary class], [NSMutableArray class], [NSNull class], nil];
+    _json = [decoder decodeObjectOfClasses:expectedClasses
+                                    forKey:kGTLRObjectJSONCoderKey];
   }
   return self;
 }

Source/Tests/GTLRObjectTest.m

@@ -292,11 +292,14 @@ - (GTLRTestingObject *)objectFromRoundTripArchiveDearchiveWithObject:(GTLRTestin
   NSMutableData *data = [NSMutableData data];
 
   NSKeyedArchiver *archiver = [[NSKeyedArchiver alloc] initForWritingWithMutableData:data];
+  [archiver setRequiresSecureCoding:YES];
   [archiver encodeObject:obj forKey:key];
   [archiver finishEncoding];
 
   NSKeyedUnarchiver *unarchiver = [[NSKeyedUnarchiver alloc] initForReadingWithData:data];
-  GTLRTestingObject *obj2 = [unarchiver decodeObjectForKey:key];
+  [unarchiver setRequiresSecureCoding:YES];
+  GTLRTestingObject *obj2 = [unarchiver decodeObjectOfClass:[obj class]
+                                                     forKey:key];
   [unarchiver finishDecoding];
 
   return obj2;
@@ -317,14 +320,16 @@ - (void)testSecureCodingEmptyObject {
 
 - (void)testSecureCoding {
   NSString * const jsonStr =
-      @"{\"a_date\":\"2011-01-14T15:00:00Z\",\"a.num\":1234,\"a_str\":\"foo bar\"}";
+      @"{\"a_date\":\"2011-01-14T15:00:00Z\",\"a.num\":1234,\"a_str\":\"foo bar\","
+      @"\"arrayString\":null}";
   NSError *err = nil;
   NSMutableDictionary *json = [self objectWithString:jsonStr
                                                error:&err];
   XCTAssertNotNil(json);
   GTLRTestingObject *obj = [GTLRTestingObject objectWithJSON:json];
   obj.userProperties = @{ @"A" : @1 };
   XCTAssertNotNil(obj);
+  XCTAssertEqual((id)obj.arrayString, (id)[NSNull null]); // pointer comparision
 
   GTLRTestingObject *obj2 = [self objectFromRoundTripArchiveDearchiveWithObject:obj];
   XCTAssertNotNil(obj2);
@@ -333,6 +338,7 @@ - (void)testSecureCoding {
   XCTAssertNotEqual(obj2.JSON, obj.JSON);
   XCTAssertNil(obj2.userProperties);  // userProperties are not encoded.
   XCTAssert([obj2.JSON isKindOfClass:[NSMutableDictionary class]]);
+  XCTAssertEqual((id)obj2.arrayString, (id)[NSNull null]); // pointer comparision
 }
 
 - (void)testSecureCodingMutability {