encoding/json: rename Indent method to SetIndent

CL 21057 added this method during the Go 1.7 cycle
(so it is not yet released and still possible to revise).

This makes it clearer that the method is not doing something
(like func Indent does), but just changing a setting about doing
something later.

Also document that this is in some sense irreversible.
I think that's probably a mistake but the original CL discussion
claimed it as a feature, so I'll leave it alone.

For #6492.

Change-Id: If4415c869a9196501056c143811a308822d5a420
Reviewed-on: https://go-review.googlesource.com/23295
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

src/encoding/json/stream_test.go

@@ -44,6 +44,9 @@ func TestEncoder(t *testing.T) {
 	for i := 0; i <= len(streamTest); i++ {
 		var buf bytes.Buffer
 		enc := NewEncoder(&buf)
+		// Check that enc.SetIndent("", "") turns off indentation.
+		enc.SetIndent(">", ".")
+		enc.SetIndent("", "")
 		for j, v := range streamTest[0:i] {
 			if err := enc.Encode(v); err != nil {
 				t.Fatalf("encode #%d: %v", j, err)
@@ -77,7 +80,7 @@ false
 func TestEncoderIndent(t *testing.T) {
 	var buf bytes.Buffer
 	enc := NewEncoder(&buf)
-	enc.Indent(">", ".")
+	enc.SetIndent(">", ".")
 	for _, v := range streamTest {
 		enc.Encode(v)
 	}