Only check if opts.body exists and doesn't have a falsy value

lib/agent.js

@@ -65,7 +65,7 @@ export default class Agent {
       opts.body = JSON.stringify(body)
     }
 
-    if (opts.body !== null && typeof body.pipe !== 'function') {
+    if (opts.body && typeof body.pipe !== 'function') {
       opts.headers['Content-Length'] = Buffer.byteLength(opts.body)
     }