Newline string literals produce invalid marshaling output

[Alfus]

If keys or values of a map node are the string literal "\n", the encoder has odd output.

func TestYamlNewLineMaps(t *testing.T) {
	value := map[string]string{
		"\n": "\n",
	}
	data, err := yaml.Marshal(value)
	if err != nil {
		t.Fatal(err)
	}
	// Should be `"\n": "\n"`, but its garbled.
	if string(data) != "? |4+\n: |4+\n" {
		t.Fatalf("Expected garbled output, got %s", string(data))
	}
}

The output is:

? |4+
: |4+

Which doesn't appear to be valid yaml. When it should be:

"\n": "\n"

[dolmen]

Here is a variation related to serialization of \n (on the Go Playground):

func TestNewLine(t *testing.T) {
	const orig = "\n"
	data, err := yaml.Marshal("\n")
	if err != nil {
		t.Fatal(err)
	}
	var s string
	err = yaml.Unmarshal(data, &s)
	if err != nil {
		t.Fatal(err)
	}
	if s != orig {
		t.Fatalf("got %q, expected %q", s, orig)
	}
}

=== RUN   TestNewLine
    prog_test.go:21: got "", expected "\n"
--- FAIL: TestNewLine (0.00s)
FAIL

[dolmen]

Related: #963, #964