Implement -[FLAnimatedImageView intrinsicContentSize]

`UIImageView` returns its image size as its `intrinsicContentSize`, but `FLAnimatedImageView` breaks this when its animatedImage property is set. This change makes `FLAnimatedImageView` much more helpful when using Auto Layout.

I'm not 100% clear why `FLAnimatedImageView`'s behaviour breaks `UIImageView`. Perhaps `UIImageView` bypasses its `image` accessor.
Could we restructure this logic a bit to avoid multiple return statements? E.g.:

```
CGSize intrinsicContentSize = [super intrinsicContentSize];

if (self.animatedImage) {
    intrinsicContentSize = self.image.size;
}

return intrinsicContentSize;
```

Or -- since the getter override `- (UIImage *)image` already does the super-class logic, could just always

```
return self.image;
```

and explain what's going on with a comment? Unless that subtly changes your logic that you have confirmed working.