Update send_form.php

Adding 'Content-Type: text/html; charset=UTF-8' in $header allows for correct display of non ASCII characters in message text. It works for me with Iceweasel, Enigmail, Debian Wheezy 64 bit. I don't know enough about PHP & email to be sure that it doesn't break anything else. Please check! :)
Merge pull request #1 from jeromecc/master

Added Content-Type send_form.php