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`.

mastodon.go

@@ -24,17 +24,12 @@ type Config struct {
 	AccessToken  string
 }
 
-type WriteResetter interface {
-	io.Writer
-	Reset()
-}
-
 // Client is a API client for mastodon.
 type Client struct {
 	http.Client
 	Config     *Config
 	UserAgent  string
-	JSONWriter WriteResetter
+	JSONWriter io.Writer
 }
 
 func (c *Client) doAPI(ctx context.Context, method string, uri string, params interface{}, res interface{}, pg *Pagination) error {
@@ -133,7 +128,6 @@ func (c *Client) doAPI(ctx context.Context, method string, uri string, params in
 	}
 
 	if c.JSONWriter != nil {
-		c.JSONWriter.Reset()
 		return json.NewDecoder(io.TeeReader(resp.Body, c.JSONWriter)).Decode(&res)
 	} else {
 		return json.NewDecoder(resp.Body).Decode(&res)