Ensure support for little endian without 'Exif\0\0' prefix

[lovell]

Little endian EXIF without the Exif\0\0 prefix is currently being ignored as the 3rd/4th bytes will be hex 2a00 rather than (big endian) 002a.

sharp v0.33.0 onwards supports EXIF in PNG, defaulting to little endian without prefix, hence this problem appearing now.

The test file was created via:

  .withExif({
    IFD0: {
      Software: "sharp"
    }
  })

This should address #34

[Profesor08]

Just patched locally with patch-package, and it work well. It woll be great to be merged and published.