& encoded to & in plain text email checksum links

& should not be encoded to & in plain text emails. In general this works OK, but when a checksum link is used in the plain text email format, the ampersand is encoded.

This is a problem because it 'breaks' the URL, which will not work when people try to click it.

Steps to recreate:

• Create a new Mailing
• Add a link which uses a checksum and has an ampersand in it, eg: https://foo.com/civicrm/profile/edit?reset=1&gid=1&id=1&cs= {contact.checksum}
• Click 'Preview as plain text'
• Note that the URL is now: https://foo.com/civicrm/profile/edit?reset=1∓gid=1&id=1&cs=123_456_789