files: Local#writeStream should use it's own file_put_contents

The OC\Files\Storage\Local#writeStream use system provided file_put_contents.
However, it overrides file_put_contents, thus expects that the default behaviour
can be different.

Use Local#file_put_contents in writeStream to benefit from class specific functionality.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>