Support "integration" tokens

Integration tokens are different than OAuth token in it that they don't report any `X-Oauth-Scopes` in response headers.

api/client.go

@@ -196,6 +196,11 @@ func (c Client) HasMinimumScopes(hostname string) error {
 	}
 
 	scopesHeader := res.Header.Get("X-Oauth-Scopes")
+	if scopesHeader == "" {
+		// if the token reports no scopes, assume that it's an integration token and give up on
+		// detecting its capabilities
+		return nil
+	}
 
 	search := map[string]bool{
 		"repo":      false,

api/client_test.go

@@ -111,6 +111,11 @@ func Test_HasMinimumScopes(t *testing.T) {
 		header  string
 		wantErr string
 	}{
+		{
+			name:    "no scopes",
+			header:  "",
+			wantErr: "",
+		},
 		{
 			name:    "default scopes",
 			header:  "repo, read:org",