notify: fix neglect of custom headers set via template

e981725da3fe introduced a bug that discarded all custom headers.
Add custom header to test template.

hgext/notify.py

@@ -185,10 +185,16 b' class notifier(object):'
 
         # store sender and subject
         sender, subject = msg['From'], msg['Subject']
+        del msg['From'], msg['Subject']
+        # store remaining headers
+        headers = msg.items()
         # create fresh mime message from msg body
         text = msg.get_payload()
         # for notification prefer readability over data precision
         msg = mail.mimeencode(self.ui, text, self.charsets, self.test)
+        # reinstate custom headers
+        for k, v in headers:
+            msg[k] = v
 
         def fix_subject(subject):
             '''try to make subject line exist and be useful.'''

tests/test-notify

@@ -52,7 +52,7 b' cat <<EOF >> $HGRCPATH'
 config = $HGTMP/.notify.conf
 domain = test.com
 strip = 3
-template = Subject: {desc|firstline|strip}\nFrom: {author}\n\nchangeset {node|short} in {webroot}\ndescription:\n\t{desc|tabindent|strip}
+template = Subject: {desc|firstline|strip}\nFrom: {author}\nX-Test: foo\n\nchangeset {node|short} in {webroot}\ndescription:\n\t{desc|tabindent|strip}
 
 [web]
 baseurl = http://test/

tests/test-notify.out

@@ -108,6 +108,7 b' added 1 changesets with 1 changes to 1 f'
 Content-Type: text/plain; charset="us-ascii"
 MIME-Version: 1.0
 Content-Transfer-Encoding: 7bit
+X-Test: foo
 Date:
 Subject: b
 From: [e-mail]
@@ -138,6 +139,7 b' added 1 changesets with 1 changes to 1 f'
 Content-Type: text/plain; charset="us-ascii"
 MIME-Version: 1.0
 Content-Transfer-Encoding: 7bit
+X-Test: foo
 Date:
 Subject: b
 From: [e-mail]