Fixed casper.getPageContent() for Gecko

Gecko encloses non HTML content in an HTML web page, like Webkit,
but with some differences.
Tests & Gecko: fixed error about createDocument in utils.js

The third parameter is mandatory in Gecko 25 and lower. It
is optional since Gecko 26 (not released yet at that time)
Reactivate some tests about http status for PhantomJS 1.9.2+
Fixed jshint issues
Merge branch 'master' into geckofix1
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Merge pull request #708 from laurentj/geckofix1

Some bug fixes for SlimerJS