url: Default the CA proxy bundle location to CURL_CA_BUNDLE

If the compile-time CURL_CA_BUNDLE location is defined use it as the
default value for the proxy CA bundle location, which is the same as
what we already do for the regular CA bundle location.

Ref: https://github.com/curl/curl/pull/1257

lib/url.c

@@ -577,6 +577,10 @@ CURLcode Curl_init_userdefined(struct UserDefined *set)
   result = setstropt(&set->str[STRING_SSL_CAFILE_ORIG], CURL_CA_BUNDLE);
   if(result)
     return result;
+
+  result = setstropt(&set->str[STRING_SSL_CAFILE_PROXY], CURL_CA_BUNDLE);
+  if(result)
+    return result;
 #endif
 #if defined(CURL_CA_PATH)
   result = setstropt(&set->str[STRING_SSL_CAPATH_ORIG], CURL_CA_PATH);