Add Buffer.WriteByte and WriteString methods (#691)

These methods are equivalent to `AppendByte` and `AppendString`, but
their signatures are compatible with
[bytes.Buffer](https://godoc.org/bytes#Buffer) and
[bufio.Writer](https://godoc.org/bufio#Writer).

This allows to use `Buffer` where `bytes.Buffer` was expected without
extra cost of wrapping. One example is msgpack library which
expects `Writer` to implement `WriteByte` and `WriteString`, otherwise
it is wrapped which incurs extra allocation:
https://github.com/vmihailenco/msgpack/blob/master/encode.go#L63-L67

buffer/buffer.go

@@ -106,6 +106,24 @@ func (b *Buffer) Write(bs []byte) (int, error) {
 	return len(bs), nil
 }
 
+// WriteByte writes a single byte to the Buffer.
+//
+// Error returned is always nil, function signature is compatible
+// with bytes.Buffer and bufio.Writer
+func (b *Buffer) WriteByte(v byte) error {
+	b.AppendByte(v)
+	return nil
+}
+
+// WriteString writes a string to the Buffer.
+//
+// Error returned is always nil, function signature is compatible
+// with bytes.Buffer and bufio.Writer
+func (b *Buffer) WriteString(s string) (int, error) {
+	b.AppendString(s)
+	return len(s), nil
+}
+
 // TrimNewline trims any final "\n" byte from the end of the buffer.
 func (b *Buffer) TrimNewline() {
 	if i := len(b.bs) - 1; i >= 0 {

buffer/buffer_test.go

@@ -48,6 +48,8 @@ func TestBufferWrites(t *testing.T) {
 		{"AppendFloat32", func() { buf.AppendFloat(float64(float32(3.14)), 32) }, "3.14"},
 		{"AppendWrite", func() { buf.Write([]byte("foo")) }, "foo"},
 		{"AppendTime", func() { buf.AppendTime(time.Date(2000, 1, 2, 3, 4, 5, 6, time.UTC), time.RFC3339) }, "2000-01-02T03:04:05Z"},
+		{"WriteByte", func() { buf.WriteByte('v') }, "v"},
+		{"WriteString", func() { buf.WriteString("foo") }, "foo"},
 	}
 
 	for _, tt := range tests {