oauth2: allow callers to pass arbitrary auth URL parameters

Many OAuth 2.0 implementations support parameters beyond those supported
by this library. This change exports a SetParam function for
constructing arbitrary key/value parameters.

Change-Id: Ice4179e7c5341bbeac8a53e389b32d59415740fa
Reviewed-on: https://go-review.googlesource.com/8054
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

oauth2_test.go

@@ -61,6 +61,15 @@ func TestAuthCodeURL(t *testing.T) {
 	}
 }
 
+func TestAuthCodeURL_CustomParam(t *testing.T) {
+	conf := newConf("server")
+	param := SetParam("foo", "bar")
+	url := conf.AuthCodeURL("baz", param)
+	if url != "server/auth?client_id=CLIENT_ID&foo=bar&redirect_uri=REDIRECT_URL&response_type=code&scope=scope1+scope2&state=baz" {
+		t.Errorf("Auth code URL doesn't match the expected, found: %v", url)
+	}
+}
+
 func TestAuthCodeURL_Optional(t *testing.T) {
 	conf := &Config{
 		ClientID: "CLIENT_ID",