Teaser splitter can create invalid markup on join

This is a consequence of this issue, since it is caused by the returns added by the splitter's Join functionality, but the cause and consequences are different, so this is a separate report.

The principal is that users make HTML mistakes, and when they do, they should be able to revert and undo the mistake without having to make additional fixes to their HTML.

First example
1. With Filtered HTML on

2. Enter this:

<p><a href="http://drupal.org/node/159981">Test</a>.</p>

'
3. Place cursor immediately after "<" in final </p> tag and press Split.

4. Result

[Top box] <p><a href="http://drupal.org/node/159981">Test</a>.</

[Bottom box] p>

5. User realizes mistake, presses Join to undo, expecting to Undo the split by rejoining what was split. Instead:

<p><a href="http://drupal.org/node/159981">Test</a>.</

p>

6. Markup is now broken. p> displays as text.

Second example
1. With Full HTML on

2. Enter this:

<p><a href="http://drupal.org/node/159981">Test</a>.</p>

3. Place cursor immediately after first quote following @href and press Split.

4. The result:

[Top box] <p><a href="

[Bottom box] http://drupal.org/node/159981">Link</a>.</p>

5. User realizes mistake, presses Join to undo, expecting to Undo the split by rejoining what was split, and then immediately Saves.

6. HTML displays as link.

7. User clicks link, expecting it to work, but link is broken because carriage returns inserted by Splitter are turned into P tags inside the URL.

http://localhost.local/drupal/node/%3C/p%3E%3Cp%3Ehttp://drupal.org/node/159981

And so on.

6.0-beta4, OS X, FF 2.0.0.11, PHP 5.2.1