Update WithHeadingFinder.php to fix heading fields for lens exports

[puzzledmonkey]

NOTE - This pull request is for the 1.2 branch because I am stuck on Nova 3 in the project I'm working on. The fix should be identical for 1.3 and 2.0 though.

When exporting from a lens, the WithHeaderFinder still uses a normal resource's index fields, instead of the lens fields. Therefore if you have any fields in the lens that are not in the index fields for the normal resource, the heading finder cannot locate them and falls back to the attribute name (eg field_name instead of Field Name)

This pull request simply uses the resourceFields (which are used for the export) to lookup the headings. This doesn't adversely affect a normal resource export because ExportResourceActionRequest uses indexFields in its resourceFields method, and it fixes ExportLensActionRequest to correctly locate the header names.

[patrickbrouwers]

Can you provide a super simple User resource (based on default users_table in Laravel) and lens example for me to test with?

[puzzledmonkey]

sure:

https://github.com/puzzledmonkey/lens-example

it should be self-explanatory (if highly contrived), but basically the issue is that, on the user lens, when downloading excel, the 'Is An Admin' column will have the header 'is_admin' instead of 'Is An Admin'. this is the issue that my patch fixes.

[puzzledmonkey]

p.s. same issue and fix for v1.3 is here: #156

[puzzledmonkey]

Can you provide a super simple User resource (based on default users_table in Laravel) and lens example for me to test with?

hey just checking if there's anything else you need from me here? thanks! :)