[iOS] S3 Issue

[odemolliens]

With current version, when we try to get an S3 image with JPG, AlamofireImage throw an error (403 - contentTypeValidationError)

Of course we can add:
Request.addAcceptableImageContentTypes(["image/jpg"])

But it seem's to be better to add it in the library (jpg is a standard format)

[cnoon]

Thanks for the PR @odemolliens. Unfortunately we won't be merging this due to image/jpg not being a standard format. You can easily add it to the list of acceptable content types though using the addAcceptableImageContentTypes API.

Please see #29 and #33 for more info.

Cheers. 🍻