Fix URL-encoding issue with port-numbers

:80 and :443 would be redundantly added to HTTP
and HTTPS URLs, respectively.

src/mirror-img.lisp

@@ -153,7 +153,7 @@ already set)."
    (format nil "~@[~A://~]~A~@[:~A~]~A"
            (quri:uri-scheme uri)
            (quri:uri-host uri)
-            (quri:uri-port uri)
+           (uri-explicit-port uri)
            (url-encode-path (quri:uri-path uri)))))
 
 ;; string → string
@@ -166,3 +166,15 @@ already set)."
       (lambda (a b)
         (format nil "~A/~A" a b))
       (mapcar #'quri:url-encode path-parts))))
+
+;; quri-uri → number
+(defun uri-explicit-port (uri)
+  "Given a HTTP/HTTPS URI, return a port-number if it isn’t implied by the protocol.
+That is, 443 is implied by HTTPS, so nil is returned; but 998 wouldn’t be implied."
+  (let ((scheme (quri:uri-scheme uri))
+        (port (quri:uri-port uri)))
+    (unless (or (and (equal scheme "https")
+                     (eq port 443))
+                (and (equal scheme "http")
+                     (eq port 80)))
+      port)))