Fixed XSS issue with link attributes

The current version of auto_link is vulnerable to a XSS attack:

```
https://www.foobar.com/"onmouseover="prompt()
```
you will get
```
<a href="https://www.foobar.com/" onmouseover="prompt()">https://www.foobar.com/"onmouseover="prompt()</a>
```

**Solution:**

The regexp should find characters until a `"` is found. Then the result is:

```
<a href="https://www.foobar.com/">https://www.foobar.com/"onmouseover="prompt()</a>onmouseover="prompt()"
```