fix(key-auth): change hide_credentials behaviour

[mfeledyn]

Summary

Change key-auth plugin "hide_credentials" parameter behaviour.

Before proposed change, "hide_credentials" parameter of key-auth plugin makes "apikey" parameter removed from both headers and query_string regardless of search locations. This very simple fix allows to remove apikey from query and/or from headers accordingly to search locations.
This is required for me as I have some API backends that rely on a "apikey" value set as header, and the Kong gateway is set to find the "apikey" in query.
Fix:

• remove parameter in query only if configured to search in query
• remove parameter in headers only if configured to search in headers

Checklist

Successfully tested.
Code change is obvious and does not require a full test spec.

Issue reference

[CLAassistant]

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