xdg-email does not parse mailto uris properly for thunderbird

When using thunderbird as mailto handler xdg-email translates mailto uris into an 'thunderbird -compose' argument. While to, cc and bcc values are properly enclosed in single quotes this is not the case for subject or body. This breaks functionality and allows to use all thunderbird -compose arguments within a mailto uri, e.g.

xdg-email 'mailto:test@example.com?subject=Test,attachment=~/.thunderbird/profiles.ini,message=/home/test/test.txt'

translates into

thunderbird -compose to='test@example.com,',subject=Test,attachment=~/.thunderbird/profiles.ini,message=/home/test/test.txt

with working attachment and message. (And, yes, ~ expands to the home directory.)

This is different from Issue 177 where the handling of attachments is intended. Here it is not.