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/test-utils_test.go

@@ -2413,7 +2413,7 @@ func uploadTestObject(t *testing.T, apiRouter http.Handler, creds auth.Credentia
 			rec = httptest.NewRecorder()
 			apiRouter.ServeHTTP(rec, req)
 			checkRespErr(rec, http.StatusOK)
-			etag := rec.Header().Get("ETag")
+			etag := rec.Header()["ETag"][0]
 			if etag == "" {
 				t.Fatalf("Unexpected empty etag")
 			}