Header escaping

@rcrichton if you could please review?
Argh, yeah I found the problem: our test is setting it up as an array https://github.com/jembi/openhim-core-js/blob/master/test/unit/messageStoreTest.coffee#L98-L100

I was building against the test, but I see now with live transactions that it's not storing the headers :/ Will fix...