http_proxy: only update upstream proxy user from credentials if user …

[mmatczuk]

…is not set

[what-the-diff]

PR Summary

• Priority Given to Proxy Flag Over Credentials Flag
  In the new update, we have made it clear that the proxy flag will be given priority over the credentials flag, if both options are specified.

• Automatic Credentials Matching with User ID
  A new safety check has been added. Now, it will first check if there is any information available in the 'User' field of the URL. In case it's unavailable, it will automatically link the credentials matching to the corresponding URL and set them as the 'User'. This promotes ease of use and reduces the risk of unattended fields.