[TIMOB-13898]  3_1_X IOS:Adding custom client header to Ti.Network XHR request

TESTING INSTRUCTION IN [JIRA](https://jira.appcelerator.org/browse/TIMOB-13898?focusedCommentId=253302&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-253302)

Backport #4309