Rename #meta_read to #read_meta

Remove unused exception variable
Add test: preserve metadata for unprocessed styles

Remove "don't allow nils..." test, which didn't test anything.
Add test for original style geometry
Make Attachement#populate_meta a private method
Less code in Attachment#read_meta
Assert original style is reprocessed
Simplify Attachment#retain_meta
Rename #retain_meta to #merge_existing_hash