Attempt to quote anyway if unquoting fails

Fixes #2356

requests/utils.py

@@ -418,10 +418,18 @@ def requote_uri(uri):
     This function passes the given URI through an unquote/quote cycle to
     ensure that it is fully and consistently quoted.
     """
-    # Unquote only the unreserved characters
-    # Then quote only illegal characters (do not quote reserved, unreserved,
-    # or '%')
-    return quote(unquote_unreserved(uri), safe="!#$%&'()*+,/:;=?@[]~")
+    safe_with_percent = "!#$%&'()*+,/:;=?@[]~"
+    safe_without_percent = "!#$&'()*+,/:;=?@[]~"
+    try:
+        # Unquote only the unreserved characters
+        # Then quote only illegal characters (do not quote reserved,
+        # unreserved, or '%')
+        return quote(unquote_unreserved(uri), safe=safe_with_percent)
+    except InvalidURL:
+        # We couldn't unquote the given URI, so let's try quoting it, but
+        # there may be unquoted '%'s in the URI. We need to make sure they're
+        # properly quoted so they do not cause issues elsewhere.
+        return quote(uri, safe=safe_without_percent)
 
 
 def address_in_network(ip, net):