Merge branch 'mjog/1126-dot-stuffing' into 'mainline'

Fix email being sent truncated after a line starting with a dot.

Closes #1126

See merge request !647

(cherry picked from commit e99115e9)

1987582b Geary.Smtp.ClientConnection: Documentation fixes
cc5d4ffa Geary.Smtp.ClientSession: Ensure SMTP data is sent dot-stuffed