Add the size of all the parts in the body.

I think it would be better to do this all in one function, instead of duplicating code in `$this->getSize()`. Something like this:

```php
    public function sendContent()
    {
        $content = '';
        foreach ($this->parts as $part) {
            $content .= "--{$this->boundary}\r\n";
            $content .= "{$part->headers}\r\n";
            $content .= $part->getContent();
            $content .= "\r\n";
        }
        $content .= "--{$this->boundary}--";
        // Finally send all the content.
        echo strlen($content) . "\r\n" . $content;
        return $this;
    }
```