- Added missing return statement
- Added missing support for SEEK_END
- Fixes #5524

To avoid having to use OCP\Appframework\Http\Http in the public - and stable
- API OCP\Appframework\Http is now both a class and a namespace.

Callers of streamCopy() expect an array to be returned containing count and result.

otherwise the returned internalPath will not match followup calls to e.g. Cache::getID()

It doesn't make sense for subclasses to have to implement
all methods.

e.g. LOCK would break with this approach

Changed method signature of relative_modified_date template method to
make it possible to add a fromTime to compare with, mostly to make it
possible to test it.

Added unit test for date and time cases.

Now only non-parable PUT requests return a stream resource.

It's easier to find errors in the code if an exception is thrown.