Fix fetching URLs with stuff that looks like HTTP headers

* lisp/url/url-http.el (url-http-parse-headers): Narrow the
buffer to the headers at the beginning to make sure
url-handle-content-transfer-encoding uses the correct
headers.  (Bug#37023)

lisp/url/url-http.el

@@ -949,6 +949,10 @@ should be shown to the user."
 	      class url-http-response-status)))
     (if (not success)
 	(url-mark-buffer-as-dead buffer)
+      ;; Narrow the buffer for url-handle-content-transfer-encoding to
+      ;; find only the headers relevant to this transaction.
+      (and (not (buffer-narrowed-p)
+                (mail-narrow-to-head)))
       (url-handle-content-transfer-encoding))
     (url-http-debug "Finished parsing HTTP headers: %S" success)
     (widen)