Use Angle brackets for In-reply-to header since it is stripped on intake.

Abhilash Rajrequested to merge
maxking/hyperkitty:in-reply-to into master

When storing the in-reply-to header, Hyperkitty strips the angle bracket but doesn't add it back when exporting the message as an mbox.

Fixes #311 (closed)

Edited by Abhilash Raj

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