Add OC_Image Unit Test

data() somehow does not return, what file_get_contents() returns,
so I skipped related tests.
Cover more cases

Use file_get_contents() and base64_encode(file_get_contents()),
to load from data and base64, to cover more cases
Optimize image and skip mimetype test
Merge branch 'master' into add_ocimage_unittest
Update skipped tests messages
Merge pull request #4283 from owncloud/add_ocimage_unittest

Add ocimage unittest