oauth2: workaround misspelling of verification_uri

Some servers misspell verification_uri as verification_url, contrary to spec RFC 8628

Example server https://issuetracker.google.com/issues/151238144

Fixes #666

Change-Id: I89e354368bbb0a4e3b979bb547b4cb37bbe1cc02
GitHub-Last-Rev: bbf169b
GitHub-Pull-Request: #667
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/527835
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Nikolay Turpitko <nick.turpitko@gmail.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Matt Hickford <matt.hickford@gmail.com>

deviceauth.go

@@ -59,6 +59,8 @@ func (c *DeviceAuthResponse) UnmarshalJSON(data []byte) error {
 	type Alias DeviceAuthResponse
 	aux := &struct {
 		ExpiresIn int64 `json:"expires_in"`
+		// workaround misspelling of verification_uri
+		VerificationURL string `json:"verification_url"`
 		*Alias
 	}{
 		Alias: (*Alias)(c),
@@ -69,6 +71,9 @@ func (c *DeviceAuthResponse) UnmarshalJSON(data []byte) error {
 	if aux.ExpiresIn != 0 {
 		c.Expiry = time.Now().UTC().Add(time.Second * time.Duration(aux.ExpiresIn))
 	}
+	if c.VerificationURI == "" {
+		c.VerificationURI = aux.VerificationURL
+	}
 	return nil
 }