Keep the API JSON around (briefly)

For my archival purposes, I like to keep the raw API JSON around
as it often contains things not picked up by the various objects.

e.g. getting a status from my Akkoma instance contains a whole
`pleroma` extension block that's not reflected in `mastodon.Status`.

Maybe this should be gated by an option when creating the `Client`
since it's probably only relevant to 1% of library users.

mastodon.go

@@ -29,6 +29,7 @@ type Client struct {
 	http.Client
 	Config    *Config
 	UserAgent string
+	LastJSON  []byte
 }
 
 func (c *Client) doAPI(ctx context.Context, method string, uri string, params interface{}, res interface{}, pg *Pagination) error {
@@ -125,7 +126,18 @@ func (c *Client) doAPI(ctx context.Context, method string, uri string, params in
 			*pg = *pg2
 		}
 	}
-	return json.NewDecoder(resp.Body).Decode(&res)
+
+	// If we want to store the JSON received, we absolutely have to
+	// read all of it.  But we restrict ourselves to a max of 100M.
+	safer := &io.LimitedReader{resp.Body, 100 * 1_048_576}
+	c.LastJSON, err = io.ReadAll(safer)
+
+	if err != nil || c.LastJSON == nil {
+		return err
+	}
+
+	// ...which means we can't use `NewDecoder.Decode` any more.
+	return json.Unmarshal(c.LastJSON, &res)
 }
 
 // NewClient returns a new mastodon API client.