Rails3: model: mailer: use .to_s at redmine_headers method

Rails3: model: mailer: use .to_s at redmine_headers method

On Rails 3.0:
<pre>
DEPRECATION WARNING: Using a non-String object for a header's value is deprecated.
You specified 15 (a Fixnum) for X-Redmine-Issue-Id.
</pre>

On Rails 3.1.3:
<pre>
Error:
test_message_posted_message_id(MailerTest):
NoMethodError: undefined method `index' for 1:Fixnum
    test/unit/mailer_test.rb:226:in `test_message_posted_message_id'
</pre>

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@8468 e93f8b46-1217-0410-a6f0-8f06a7374b81