With the version of mjml we use, trailing text nodes get lost at rendering.
Example: in "<strong>Client</strong>: ACME", the ": ACME" part will be lost.
To avoid this issue, we automatically add a <span/> around such tail text nodes.

The right solution is to update mjml, but it seems to resist me for some
reasons. Until then, we work around.

This version fixes the case were tags are splitted among several xml elements