mime/quotedprintable: add examples

Partially addresses #16360

Change-Id: Ic098d2c465742fb50aee325a3fd0e1d50b7b3c99
Reviewed-on: https://go-review.googlesource.com/25575Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/mime/quotedprintable/example_test.go

+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quotedprintable_test
+
+import (
+	"fmt"
+	"io/ioutil"
+	"mime/quotedprintable"
+	"os"
+	"strings"
+)
+
+func ExampleNewReader() {
+	for _, s := range []string{
+		`=48=65=6C=6C=6F=2C=20=47=6F=70=68=65=72=73=21`,
+		`invalid escape: =B`,
+		"Hello, Gophers! This symbol will be unescaped: =3D and this will be written in =\r\none line.",
+	} {
+		b, err := ioutil.ReadAll(quotedprintable.NewReader(strings.NewReader(s)))
+		fmt.Printf("%s %v\n", b, err)
+	}
+	// Output:
+	// Hello, Gophers! <nil>
+	// invalid escape:  unexpected EOF
+	// Hello, Gophers! This symbol will be unescaped: = and this will be written in one line. <nil>
+}
+
+func ExampleNewWriter() {
+	w := quotedprintable.NewWriter(os.Stdout)
+	w.Write([]byte("These symbols will be escaped: = \t"))
+	w.Close()
+
+	// Output:
+	// These symbols will be escaped: =3D =09
+}