form-data >= 2.3.2 fragments data, causing delays #422

If you run the following with form-data v2.3.2:

const form = new (require("form-data"))()
form.append("foo", "Some text")
form.append("bar", "Hmmm")
form.submit("http://example.com")

You will see that it first sends an incomplete chunk of data to the server:

POST / HTTP/1.1
content-type: multipart/form-data; boundary=--------------------------602269624292240232678615
Host: example.com
Content-Length: 273
Connection: close

----------------------------602269624292240232678615
Content-Disposition: form-data; name="foo"

Some text

After that, Nagle's algorithm kicks in and blocks the remaining data until the server sends an ACK. Then it's sent:

 
----------------------------602269624292240232678615 
Content-Disposition: form-data; name="bar" 
 
Hmmm 
----------------------------602269624292240232678615-- 

In conclusion, all requests made with form-data have at least 1 RTT of delay added to them. This may not seem much (146ms for me), but for servers that are far away and/or lots of requests, it has a substantial impact.

We have tracked this down to felixge/node-combined-stream#38. Before that PR, data chunks were written synchronously and Node.JS could buffer them into a single write to the socket. After the change, buffering is not possible.