Upgrade PHPMailer from 5.2.1 to 5.2.6

See changelog.md for details

Fixes #14543, #15953
soap tests: fix whitespace
soap/VersionTest.php: take timezone into consideration

Prior to this, dates were compared as string, resulting in
'2015-10-29T12:59:14+00:00' and '2015-10-29T05:59:14-07:00' to be
considered as different dates.

Fixes #15817
soap/AttachmentTest.php: fix reference download URL

mc_issue_api.php/mci_issue_get_attachments() generates the download URL
with '&amp;' while the test suite compares it with '&'. In some cases
the soap api returns '&' while in most cases it returns '&amp;'.

To fix the problem we decode the string with html_entity_decode() before
comparing it.

Fixes #15817
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