email signature not included in activity email

to reproduce:

• go into your contact record and click the edit button
• below the email address click to expand the signature panel. enter a value and save the form.
• on the summary view you will now see "(signature)" in parens and can click to view the value
• click the actions button and select "send an email"

the signature should prepopulate in the email body field but does not.

this is a regression. the behavior works as expected in 4.6 but was lost somewhere in the 4.7/5.x cycle.