Support 409 conflict for post requests

[ordnungswidrig]

While RFC2626 suggests only PUT requests to return 409, this can also be useful for POST requests.

See https://groups.google.com/forum/#!topic/clojure-liberator/RmR1RzIxBh0

[aperiodic]

+1. We use POSTs to update resources, but sometimes the edits don't make sense, so it would be useful to return a 409 after executing post!.

[ordnungswidrig]

If the edits don't make sense, then 422 unprocessable entity might be a better choice.

[rlinehan]

+1. I've got basically the exact same problem as in this post: https://groups.google.com/forum/#!msg/clojure-liberator/iaxFQPMZqJs/bMETIFMLHwwJ - the email address in our user table must be unique, and a 409 should be returned for a POST to create a user with an email address that already exists in the table.

[ttasterisco]

+1, similar use case as @rlinehan's

[vitillo]

+1, similar use case as @rlinehan's

[conan]

+1, also similar to @rlinehan's

[sethkrasnianski]

+1

[ordnungswidrig]

I'm open for a PR. I suggest to support conflict for POST to move the decision "conflict?" be before "post-to-existing?" (see the graph for reference: http://clojure-liberator.github.io/liberator/assets/img/decision-graph.svg). After that change conflict? will be called for POST and PUT requests.