Revert to simple io.Writer with client-resetting

After going through several other solutions, there's no clean way
to use an `io.Writer` but also be able to call `Reset()` between
writes.  Filehandles can't `Reset()` and buffers can't `Seek()`.
But then if you supplied `os.Stderr`, you'd be expecting appending
and a `Seek()` would be nonsense.

My own preference would be to make `JSONWriter` a strict `*bytes.Buffer`
which lets the library handle the resetting and the client do any
outputting if required - I can't see much value in ever supplying
a non-`bytes.Buffer` as `JSONWriter`.

status_test.go

@@ -1,6 +1,7 @@
 package mastodon
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"io/ioutil"
@@ -52,7 +53,8 @@ func TestGetFavouritesSavedJSON(t *testing.T) {
 		AccessToken:  "zoo",
 	})
 
-	client.SaveJSON = true
+	var buf bytes.Buffer
+	client.JSONWriter = &buf
 
 	favs, err := client.GetFavourites(context.Background(), nil)
 	if err != nil {
@@ -70,7 +72,7 @@ func TestGetFavouritesSavedJSON(t *testing.T) {
 
 	// We get a trailing `\n` from the API which we need to trim
 	// off before we compare it with our literal above.
-	theirJSON := strings.TrimSpace(string(client.LastJSON))
+	theirJSON := strings.TrimSpace(string(buf.Bytes()))
 
 	if theirJSON != ourJSON {
 		t.Fatalf("want %q but %q", ourJSON, theirJSON)