Authkey is currently not sent for WMS GetLegendGraphic requests

[tdipisa]

Description

Not sure it is a regression or a new bug, but it seems the authkey is not sent with MWS GetLegendGraphic requests.

How to reproduce

• Open the legend of a layer as authenticated user

Expected Result

The authkey param is sent as query parameter in WMSGetLegendGraphic requests performed by MS

Current Result

The authkey is not included

• Not browser related

Browser info

(use this site: https://www.whatsmybrowser.org/ for non expert users)

Browser Affected	Version
Internet Explorer
Edge
Chrome
Firefox
Safari

Other useful information

this is due to this if which is not true and there fore the authkey is not added. we should add this SLD when the requests comes from GetLegendGraphich

8b44c0e#diff-c8a7cdc28e2e637335975a5a48e6aafc95c21dd42f99d30d3714e2d4423f77a1R625

we can add an if clause here for authkey that adds it if the method is authkey
https://github.com/geosolutions-it/MapStore2/blob/master/web/client/components/misc/SecureImage.jsx#L34

DO NOT alter code for addAuthenticationToSLD, but prefer to use addAuthenticationParameter in SecureImage