Filesystem client: only return requested attributes #33
Description
The filesystem client returns the entire indexed pydicom.Dataset in all cases for search methods like series_for_study and studies_for_patient.
This can cause clients to rely on the presence of unrequested attributes (i.e. not in additional_tags), which will then be missing when using a real DICOM source with DcmtkClient or PynetDicomClient.
copy_dicom_attributes should be used in conjunction with default tags used in DcmtkClient/PynetDicomClient to only copy requested tags, more closely matching real-world behavior.