Allow binding `url-mime-accept-string'

* lisp/url/url-http.el (url-http): Allow binding
`url-mime-accept-string' (bug#22855).

Backport:

(cherry picked from commit 144bb0cf32)

etc/NEWS

@@ -927,6 +927,10 @@ a function.
 to specify that we're running in a noninteractive context, and that
 we should not be queried about things like TLS certificate validity.
 
+---
+*** `url-mime-accept-string' can now be used as in "interface"
+variable, meaning you can bind it around an `url-retrieve' call.
+
 ---
 *** If URL is used with a https connection, the first callback argument
 plist will contain a :peer element that has the output of

lisp/url/url-http.el

@@ -1213,6 +1213,7 @@ overriding the value of `url-gateway-method'."
 				 (and (boundp 'url-http-noninteractive)
 				      url-http-noninteractive)))
 	 (connection (url-http-find-free-connection host port gateway-method))
+         (mime-accept-string url-mime-accept-string)
 	 (buffer (or retry-buffer
 		     (generate-new-buffer
                       (format " *http %s:%d*" host port)))))
@@ -1248,6 +1249,7 @@ overriding the value of `url-gateway-method'."
 		       url-http-target-url
 		       url-http-no-retry
 		       url-http-connection-opened
+                       url-mime-accept-string
 		       url-http-proxy))
 	  (set (make-local-variable var) nil))
 
@@ -1265,6 +1267,7 @@ overriding the value of `url-gateway-method'."
 	      url-http-target-url url-current-object
 	      url-http-no-retry retry-buffer
 	      url-http-connection-opened nil
+              url-mime-accept-string mime-accept-string
 	      url-http-proxy url-using-proxy)
 
 	(set-process-buffer connection buffer)