Add content-length header with form-data

[item4]

Old source do not add content-length header automatically
with form-data. If body is instance of form-data, Users
must missing this header. It cause parse error with some
server engine like WSGI. WSGI must drop body if this was not
set, and WSGI based frameworks such as flask can not read
form data.

For example, this python code with flask..

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/', method=['POST'])
def index():
    return jsonify(**request.form)

app.run()

Can not receive any data from this code.

require('es6-promise').polyfill();
var fetch = require('isomorphic-fetch'); // it use node-fetch
var FormData = require('form-data');

var data = new FormData();
data.append('name', 'item4');
data.append('message', 'it is test!');
fetch('http://localhost:5000', {method: 'POST', body: data})
  .then(function (data) {
    return data.json();
  })
  .then(function (data) {
    console.log(data);
  });

This commit just use FormData's getLengthSync method.

[bitinn]

I want to double check one thing, if you do manually pass headers: form.getHeaders() to fetch, it works right? Does form-data set content-length by itself?

[item4]

So sadly, No.

See this implementation.

[olalonde]

Having same issue, fetch doesn't set Content-Length header.

[bitinn]

@olalonde yep I should merge this...

[bitinn]

And in case people need an alternative for passing custom headers with form-data, getCustomHeaders seems like a good solution.

https://github.com/form-data/form-data/blob/master/lib/form_data.js#L272

[dbo]

@bitinn Any chance to get a new official release npm? The last one is 5 months old and 3 PRs have been landing meanwhile on master.