api: Sent ErrPreconditionFailed on If-Match failure (#2009)

* api: Sent ErrPreconditionFailed on If-Match failure

ref:
http://docs.aws.amazon.com/AmazonS3/latest/API/ErrorResponses.html#ErrorCodeList

* tests: Added functional tests for GetObject w/ If-Match headers set

* tests: Used verifyError to simplify errorCode and description matching on error

object-handlers.go

@@ -242,7 +242,7 @@ func checkETag(w http.ResponseWriter, r *http.Request) bool {
 			delete(h, "Content-Type")
 			delete(h, "Content-Length")
 			delete(h, "Content-Range")
-			w.WriteHeader(http.StatusPreconditionFailed)
+			writeErrorResponse(w, r, ErrPreconditionFailed, r.URL.Path)
 			return true
 		}
 	}