Make it possible to enumetate all keys in POSJSONMap (NSFastEnumeration)

[Pavel Osipov]

Use case example.

@implementation POSJSONMap (MRCClimate)

- (NSArray<NSString *> *)allKeys {
    return [self map:^id _Nonnull(NSString * _Nonnull key, POSJSONObject * _Nonnull value) {
        return key;
    }];
}

@end