fix(fetch): Explicitly check for FormData type (nodejs#1451)

Author: timdp

lib/fetch/body.js

@@ -71,7 +71,7 @@ function extractBody (object, keepalive = false) {
 
     // Set source to a copy of the bytes held by object.
     source = new Uint8Array(object)
-  } else if (util.isFormDataLike(object)) {
+  } else if (object instanceof FormData || util.isFormDataLike(object)) {
     const boundary = '----formdata-undici-' + Math.random()
     const prefix = `--${boundary}\r\nContent-Disposition: form-data`