mediatype application/json to strict

[GoogleCodeExporter]

jsonovich is only supporting application/json, the standard has vendor specific 
subtypes like
application/vnd.example+json

Original issue reported on code.google.com by da...@graveyard.dk on 16 Oct 2012 at 7:54

[welwood08]

Since there is no longer the above-mentioned platform restriction for exact matches in the webext implementation, this should be possible now.