diff -r d3a9036d9ae9 -r a708e1e4d7a8 mercurial/debugcommands.py
--- a/mercurial/debugcommands.py	Tue Mar 13 11:20:07 2018 -0700
+++ b/mercurial/debugcommands.py	Tue Mar 13 10:34:36 2018 -0700
@@ -2846,6 +2846,12 @@
         if ui.debugflag:
             openerargs[r'loggingopts'][r'logdataapis'] = True
 
+        # Don't send default headers when in raw mode. This allows us to
+        # bypass most of the behavior of our URL handling code so we can
+        # have near complete control over what's sent on the wire.
+        if opts['peer'] == 'raw':
+            openerargs[r'sendaccept'] = False
+
         opener = urlmod.opener(ui, authinfo, **openerargs)
 
         if opts['peer'] == 'raw':