Handle non-standard but still in use image/x-jpeg mimetype as JPEG

[dvlpp]

I stumbled upon this issue for the second time recently, so I decided to try to submit a PR: I sometimes have to deal with image/x-jpeg files (seems that it's a non-standard mime type, but some use it...).

Without this PR, the code breaks with "image/x-jpeg" is not a valid backing value for enum Intervention\Image\MediaType

[olivervogel]

Thanks for the PR. You're right, the non-standard type should be included. Can you please fix the erroneous tests, then I can merge this as is.

[olivervogel]

Thanks.