fix(storage): complete fix for PostPolicy fields (#2887)

In #2769 we neglected to add these conditions to the returned
policyFields that the end user uses to populate the request. This
resulted in an integration test failing. This PR fixes the issue.

Fixes #2882

storage/integration_test.go

@@ -3389,7 +3389,7 @@ func TestIntegration_PostPolicyV4(t *testing.T) {
 	}
 	if g, w := res.StatusCode, statusCodeToRespond; g != w {
 		blob, _ := httputil.DumpResponse(res, true)
-		t.Errorf("Status code in response mismatch: got %d want %d\nBody: %s", g, w, blob)
+		t.Fatalf("Status code in response mismatch: got %d want %d\nBody: %s", g, w, blob)
 	}
 	io.Copy(ioutil.Discard, res.Body)
 

storage/post_policy_v4.go

@@ -267,8 +267,12 @@ func GenerateSignedPostPolicyV4(bucket, object string, opts *PostPolicyV4Options
 		"x-goog-date":             now.Format(iso8601),
 		"x-goog-credential":       opts.GoogleAccessID + "/" + YYYYMMDD + "/auto/storage/goog4_request",
 		"x-goog-algorithm":        "GOOG4-RSA-SHA256",
-		"success_action_redirect": descFields.RedirectToURLOnSuccess,
 		"acl":                     descFields.ACL,
+		"cache-control":           descFields.CacheControl,
+		"content-disposition":     descFields.ContentDisposition,
+		"content-encoding":        descFields.ContentEncoding,
+		"content-type":            descFields.ContentType,
+		"success_action_redirect": descFields.RedirectToURLOnSuccess,
 	}
 	for key, value := range descFields.Metadata {
 		conds = append(conds, &singleValueCondition{key, value})