Self HREFs are not always absolute

[gadomski]

In the documentation for STACObject.get_self_href, it states that the returned HREF will be absolute:

pystac/pystac/stac_object.py

Lines 145 to 164 in 76ef6a3

	def get_self_href(self) -> Optional[str]:
	"""Gets the absolute HREF that is represented by the ``rel == 'self'``
	:class:`~pystac.Link`.
	Returns:
	str or None: The absolute HREF of this object, or ``None`` if
	there is no self link defined.
	Note:
	A self link can exist for objects, even if the link is not read or
	written to the JSON-serialized version of the object. Any object
	read from :func:`STACObject.from_file <pystac.STACObject.from_file>` will
	have the HREF the file was read from set as it's self HREF. All self
	links have absolute (as opposed to relative) HREFs.
	"""
	self_link = self.get_single_link(pystac.RelType.SELF)
	if self_link and self_link.has_target_href():
	return self_link.get_target_str()
	else:
	return None

However, no work is done to ensure that the self href is absolute, as demonstrated by this failing test:

def test_relative_self_href(self) -> None:
    self.item.add_link(pystac.Link("self", "a/relative/path/item.json"))
    # this fails
    self.assertEqual(
           self.item.get_self_href(), os.path.abspath("a/relative/path/item.json")
    )

This is true for objects read via, e.g. pystac.read_file as well. Should the returned self HREF be resolved to an absolute HREF as best as possible, or should we just return None?

[lossyrob]

Fixed by #574