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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To sum up the situation, the following config options are deprecated:

In aria.core.IO:
- `useXHRHeader` and `defaultXHRHeader` (use `headers` and `postHeaders`)
- `defaultPostHeader`, `useDefaultPostHeader`, `defaultContentTypeHeader`,
   `useDefaultContentTypeHeader` (use `headers` and `postHeaders`)

In aria.modules.RequestBeans:RequestObject:
- `postHeader` (use `headers`)

In aria.core.CfgBeans:IOAsyncRequestCfg:
- `postHeader`, `contentTypeHeader` (use `headers`)

Close #1044.