refactor location of @link_headers assignment
refactor location of @link_headers assignment

`csvw_validation_tests.feature:1046` and `csvw_validation_tests.feature:1075` were failing because link headers was left unpopulated due to empty hash assignment in Initialise (added to catch previously failing tests).
It has been added to the `validate_metadata` section where @headers is guaranteed to have been initialised to not nil

theodi/csvlint.rb,strategicdata/csvlint.rb,nickzoic/csvlint.rb,strategicdata/csvlint.rb,ResultadosDigitais/csvlint.rb