fix(storage): encode + in SignedURL query params (#2895)

This must be encoded for the signature to be correct if a
query parameter contains a plus sign. We have a conformance
test case for this which can now be unskipped.

Fixes #2876

storage/conformance_test.go

@@ -151,9 +151,6 @@ func TestSigningV4Conformance(t *testing.T) {
 	for _, testFile := range testFiles {
 		for _, tc := range testFile.SigningV4Tests {
 			t.Run(tc.Description, func(t *testing.T) {
-				if tc.Description == "Query Parameter Encoding" {
-					t.Skip("https://github.com/googleapis/google-cloud-go/issues/2876")
-				}
 				utcNow = func() time.Time {
 					return time.Unix(tc.Timestamp.Seconds, 0).UTC()
 				}

storage/storage.go

@@ -587,8 +587,10 @@ func signedURLV4(bucket, name string, opts *SignedURLOptions, now time.Time) (st
 	for k, v := range opts.QueryParameters {
 		canonicalQueryString[k] = append(canonicalQueryString[k], v...)
 	}
-
-	fmt.Fprintf(buf, "%s\n", canonicalQueryString.Encode())
+	// url.Values.Encode escaping is correct, except that a space must be replaced
+	// by `%20` rather than `+`.
+	escapedQuery := strings.Replace(canonicalQueryString.Encode(), "+", "%20", -1)
+	fmt.Fprintf(buf, "%s\n", escapedQuery)
 
 	// Fill in the hostname based on the desired URL style.
 	u.Host = opts.Style.host(bucket)