chore: remove isJSON in res.length (#1399)

lib/response.js

@@ -9,7 +9,6 @@ const contentDisposition = require('content-disposition');
 const ensureErrorHandler = require('error-inject');
 const getType = require('cache-content-type');
 const onFinish = require('on-finished');
-const isJSON = require('koa-is-json');
 const escape = require('escape-html');
 const typeis = require('type-is').is;
 const statuses = require('statuses');
@@ -20,6 +19,7 @@ const vary = require('vary');
 const only = require('only');
 const util = require('util');
 const encodeUrl = require('encodeurl');
+const Stream = require('stream');
 
 /**
  * Prototype.
@@ -201,18 +201,15 @@ module.exports = {
    */
 
   get length() {
-    const len = this.header['content-length'];
-    const body = this.body;
-
-    if (null == len) {
-      if (!body) return;
-      if ('string' == typeof body) return Buffer.byteLength(body);
-      if (Buffer.isBuffer(body)) return body.length;
-      if (isJSON(body)) return Buffer.byteLength(JSON.stringify(body));
-      return;
+    if (this.has('Content-Length')) {
+      return parseInt(this.get('Content-Length'), 10) || 0;
     }
 
-    return Math.trunc(len) || 0;
+    const { body } = this;
+    if (!body || body instanceof Stream) return undefined;
+    if ('string' === typeof body) return Buffer.byteLength(body);
+    if (Buffer.isBuffer(body)) return body.length;
+    return Buffer.byteLength(JSON.stringify(body));
   },
 
   /**