MailFormatHelper::htmlToText() incorrect handling of newlines in anchor links

Problem/Motivation

MailFormatHelper::htmlToText() does not handle the situation where there are newlines in the link text. For example, given the test sequence:

$html_text = "<a href=\"http://some.url\">Link \ntext</a>";
$plain_text = drupal_html_to_text($html_text);

We get:

Link text

when we should get:

Link text [1]

[1] http://some.url

Steps to reproduce

Proposed resolution

Fix the regex to strip \n from anchor text.

Remaining tasks

1. Patch
2. Test
3. Review
4. Commit