text version of header/footer not used when auto-generating text version of email

• in the header/footer config we can supply content for text and html
• IF you construct an email and explicitly provide content for the text version, then the email is constructed using the text version of the header/footer (along with the provided text version of the body content)
• IF you do not provide text content, CivicRM takes the fully constructed html version of the email (header + body + footer) and creates a text-friendly version from that.

the expected behavior in that case is to use the text header + converted-html-body + text footer

ref: http://dev.nysenate.gov/issues/9853