Correctly read the full stream in file_get_contents

When using user-defined stream wrappers, PHP will
return a maximum of 8192 bytes even if more was
requested.

This fix uses stream_get_contents to make sure the full stream is read
and not only the first 8 KB.

Added unit test with a bigger test file to cover this case.
Added missing fclose in file_get_contents and file_put_contents
Merge pull request #7979 from owncloud/core-fixfilegetcontentsloop

Correctly read the full stream in file_get_contents