Adding support for If-Unmodified-Since

[drsnyder]

WIP

This pull request adds support for the If-Unmodified-Since header to the PUT and POST methods.

• [x] verify If-Unmodified-Since header is always lowercase in the handler
• [x] document the behavior of If-Unmodified-Since in the README and the gotchas.
• [x] if the If-Unmodified-Since header is not specified, accept the update unconditionally
• [x] how do we handle the wrong date format?
• [x] remove nested conditional here

/cc @loganlinn

[loganlinn]

functionally lgtm