Fix dialyzer spec and documentation for Image.exif/1

[ntodd]

No description provided.

[kipcole9]

@ntodd thanks very much for the PR, greatly appreciated. Sorry it took so long to merge - somehow the notification scrolled past me.

I am to be publishing a new release this weekend and your PR will be included. Thanks again.

[kipcole9]

The changelog entry now reads:

Bug Fixes

• Fix specs for Image.Options.Write. Thanks to @jarrodmoldrich. Closes Dialyzer error #36.

• Fix spec for Image.exif/1. Thanks to @ntodd for the PR. Closes Fix dialyzer spec and documentation for Image.exif/1 #35.

Enhancements

• Image.Options.Write.validate_options/2 now validates options appropriate to each image type in order to make validation more robust.

• Adds :minimize_file_size option to Image.write/2 for JPEG and PNG files which if true will apply a range of techniques to minimize the size of the image file at the expense of time to save the image and potentially image quality.