Enhance: elastic.lua getHeadersReverse/getHeaders could share code

The code for getHeadersReverse/getHeaders is basically the same apart from the sort field (but see #146) and the call to table.insert()

The code could be shared by using a common function that accepted a direction parameter.

Start in elastic.lua by comparing getHeadersReverse and getHeaders, especially their sort-field handling and table.insert() calls. Extract the shared behavior while preserving the differing direction and insertion behavior, then verify that both functions still return the expected headers.

Written by the indexing model from the issue text.