Preserve ETag case for S3 compatibility (#7498)

Most hadoop distributions hortonworks, cloudera all
depend on aws-sdk-java 1.7.x to 1.10.x - the releases
which have bugs related case sensitive check for
ETag header. Go changes the case of the headers set
to be canonical but only preserves them when set
through a direct map.

This fixes most compatibility issues we have had
in the past supporting older hadoop distributions.

cmd/bucket-handlers.go

@@ -669,7 +669,7 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 	}
 
 	location := getObjectLocation(r, globalDomainNames, bucket, object)
-	w.Header().Set("ETag", `"`+objInfo.ETag+`"`)
+	w.Header()["ETag"] = []string{`"` + objInfo.ETag + `"`}
 	w.Header().Set("Location", location)
 
 	// Notify object created event.