Fixes #2422 .

Description

This PR added support to use original pixdim in Spacing transform, also fixed a minor issue when using 1 single float number for all the dims. According to the test case:
https://github.com/Project-MONAI/MONAI/blob/dev/tests/test_spacing.py#L151
it should expand the float number to all dims, but we actually used 1.0 or the original pixdim to pad for this case:
https://github.com/Project-MONAI/MONAI/blob/dev/monai/transforms/spatial/array.py#L174

Status

Ready

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.