GitHub actions fix (#29)

* Ignoring file_sizes output for tests

Different versions of exiftool and different operating systems produce various results for the file size tests.

Running locally and on remote systems like Travis-CI and GitHub actions produces different result.

for example:
        Expected [:file_size]: "24 KiB"
        Actual [:file_size]: "25 kB"

test/exiftool_test.rb

@@ -17,12 +17,14 @@
   file_modify_date
   file_modify_date_civil
   file_permissions
+  file_size
   intelligent_contrast
   max_focal_length
   min_focal_length
   source_file
   thumbnail_image
   preview_tiff
+
 ].freeze
 
 IGNORABLE_PATTERNS = [