strcase: fix raw lowercasing the letter X

Casing mistake in Curl_raw_tolower 'X' wasn't lowercased as 'x' prior to
this change.

Follow-up to 0023fce which added the function several days ago.

Ref: https://github.com/curl/curl/pull/4401#discussion_r327396546

Closes https://github.com/curl/curl/pull/4408

lib/strcase.c

@@ -150,7 +150,7 @@ char Curl_raw_tolower(char in)
   case 'W':
     return 'w';
   case 'X':
-    return 'X';
+    return 'x';
   case 'Y':
     return 'y';
   case 'Z':