Missing `Content-Length` in headers may cause nginx 411 Length Required Error.

I’m not sure this is a bug, but content-length should be prepared in headers so that server knows when the chunk datas end.

I fixed this in my own project about uploading a file by calling getLength() in headers, but this cause a lot time when sending a big file.

As I know, chrome added content-length before sending the whole post content.

[yyfrankyy]

Nothing special, this is my solution:

      var f = new formData()
      f.append('uploadfile', fs.createReadStream(file))
      f.append('formId', 1)
      f.getLength(function(err, length) {
        var token = qs.parse(url.parse(body.ErrMsg).query).token
        request.post({
            url: '/path/'
          , headers: {
              'content-length': length // 26xxxx
            }
        }, function(err, res, body) {
          var formId = body.match(/formId, '(\d+)'/)
          cb(err, formId && formId[1])
        })._form = f
      })

Here goes getLengthSync() version(wrong length)

      var f = new formData()
      f.append('uploadfile', fs.createReadStream(file))
      f.append('formId', 1)
      request.post({
          url: wx('/cgi-bin/uploadmaterial?cgi=uploadmaterial&type=0&t=iframe-uploadfile&lang=zh_CN&formId=1', token)
        , headers: {
            'content-length': f.getLengthSync() // 214
          }
      }, function(err, res, body) {
        var formId = body.match(/formId, '(\d+)'/)
        cb(err, formId && formId[1])
      })._form = f

[alexindigo]

@yyfrankyy I updated Readme.