add image/webp to mime.types for tests on 12.04
add image/webp to mime.types for tests on 12.04

closes #1285

webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed