date metadata is ignored

[masukomi]

If you specify a date in the subject's metadata 2 problems will arise

1. two dates will show up in the YAML Frontmatter (the date the post was received and later the date specified in the subject)
2. image files are saved in a date folder corresponding to the date of the post not the date specified in the metadata

Solutions: Once the post is deemed to not be spam look for a specified date. If it is found, parse it into a DateTime object so that we can use it for the image folder. Add an unless around the auto-generated date key for the YAML frontmatter that skips it if the user has specified one.