feat #1044 add aria.core.IO.postHeaders

This commit adds `aria.core.IO.postHeaders` array in order to add more
flexibility to the users and facilitate code simplification in 1.6.1.

It also properly deprecates certain features intially deprecated in 1.2.6
and 1.3.1 by improving documentation and adding deprecation warnings.

The code has been also slightly refactored to facilitate removal of
deprecated items in 1.6.1.

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This branch is based on 1052 which will be integrated soon (in order to not unnecessarily rebase).
The commit is **backward compatible** and can land in master once reviewed.

The removal commit   https://github.com/ariatemplates/ariatemplates/pull/1057 is to be integrated in 1.6.1.