[S3] Support for http caching headers

[mwhooker]

I see two issues with supporting this currently

• No easy way to construct http request with cache parameters
• poor support for 304 responses

Right now I have to call GetResponseWithHeaders with my own header object. I suppose this is probably fine, but it's a bit of boiler plate code that the library could provide.

If I construct a request with an If-None-Match header and receive a 304 response, GetResponseWithHeaders will return a nil value for the response and an error with the 304 status code. It would be ideal if I had access to the response body so I could inspect resp.StatusCode instead of parsing the error

[korya]

+1.

It does not make sense at all to return an error for non-error HTTP status code; that is, if the status code is 2xx or 3xx, there should be no error, and a caller should get the response object.

Additional problem is that this logic is hidden in a very low level API, doHttpRequest() (https://github.com/AdRoll/goamz/blob/master/s3/s3.go#L1162), making it really hard to find a workaround.