Add warnings for technically incorrect status codes

[untitaker]

Continuing #38

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See https://tools.ietf.org/html/rfc7231#section-4.3.4

Money quote:

If the target resource does not have a current representation and the
PUT successfully creates one, then the origin server MUST inform the
user agent by sending a 201 (Created) response.  If the target
resource does have a current representation and that representation
is successfully modified in accordance with the state of the enclosed
representation, then the origin server MUST send either a 200 (OK) or
a 204 (No Content) response to indicate successful completion of the
request.