Fix form method

[simov]

Fixes #1630

It turned out that when the body is set via the .form() method the content-length is not being set, and therefore the request is sent as transfer-encoding:'chunked' which may cause problems with some servers.

So I refactored out the logic responsible for setting up the content-length:

function setContentLength () {
  if (!Buffer.isBuffer(self.body) && !Array.isArray(self.body)) {
    self.body = new Buffer(self.body)
  }
  if (!self.hasHeader('content-length')) {
    var length = (Array.isArray(self.body))
      ? self.body.reduce(function (a, b) {return a + b.length}, 0)
      : self.body.length
    if (length) {
      self.setHeader('content-length', length)
    } else {
      self.emit('error', new Error('Argument error, options.body.'))
    }
  }
}

Much sexier than it was before, plus that conditional structure allows it to be called again inside the defer method. Now for the function name and declaration position it's just there for now, I couldn't figure out a better place to put it, but then again it's visible inside the defer function.

Lastly the test-form-urlencoded tests now checks if the content-length header is present as well.