[core][parser] Fix flaws in textual value encoding with in-memory time representations

This fixes two bugs which affect the writing of DICOM data when a time or date-time primitive value is constructed in-memory as a binary value (for example, using DicomDateTime).

[core] Fix reported length of encoded datetime values
[parser] Use the right padding byte when encoding a DICOM value as text when it is not already in its textual form in-memory

Also added more test coverage in the object crate.

Resolves #311.

Enet4 / dicom-rs

[core][parser] Fix flaws in textual value encoding with in-memory time representations #309