encoding/xml: make sure Encoder.Encode reports Write errors.

Fixes #4112.

R=remyoudompheng, daniel.morsing, dave, rsc
CC=golang-dev
https://golang.org/cl/7085053

src/pkg/encoding/xml/xml_test.go

@@ -689,3 +689,17 @@ func TestDirectivesWithComments(t *testing.T) {
 		}
 	}
 }
+
+// Writer whose Write method always returns an error.
+type errWriter struct{}
+
+func (errWriter) Write(p []byte) (n int, err error) { return 0, fmt.Errorf("unwritable") }
+
+func TestEscapeTextIOErrors(t *testing.T) {
+	expectErr := "unwritable"
+	err := EscapeText(errWriter{}, []byte{'A'})
+
+	if err == nil || err.Error() != expectErr {
+		t.Errorf("EscapeTest = [error] %v, want %v", err, expectErr)
+	}
+}