Fix bugs with the mobify tag being placed above the opening head tag in order browsers

**Status**: Review
**Review**: @noahadams 

## Changes
- In older browsers (Safari 4/iOS 4.3), placing the tag above opening `<head>` causes issues. To fix this, we must properly grab the `headContent` when splitting up the plaintext string.
 
## How to test-drive this PR
- Download Safari 4.0.5 (http://mirror.jquery.com/multi-safari/multi-safari-4.0.5-snow-leopard.zip)
- Run the capture.html test on Safari 4.0.5, on Chrome, on Phantomjs (`grunt test`) and on Saucelabs (`grunt saucelabs`), and Android 2.3