make it possible to influence output type of \OC_Image

Conflicts:
	lib/private/image.php
always output a png
don't change mimetype of whole image object
remove image/pjpeg from OC_Image::_output
OC_Image::_output() - throw exception instead of falling back to png
excerpt code changes from 79ba930ef90d3e54c02f62c43f61c190e11fb4ec
unit tests for specific image type output added
Merge pull request #10645 from owncloud/backport_7728

Backport 7728