asset download: save memory, use readfile => bugfix, headers

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@3889 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
build auto version (2962) ...

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@3890 1f8fe7d8-47f0-464c-8d0a-336f4953ab05