`jsonld.expand` incorrect when there are identical scoped contexts about pyld

[rossbowen]

An issue looks to be here:

pyld/lib/pyld/jsonld.py

Lines 5510 to 5515 in 316fbc2

	if base and '@base' in active_ctx:
	# The None case preserves rval as potentially relative
	if active_ctx['@base'] is not None:
	rval = prepend_base(prepend_base(base, active_ctx['@base']), rval)
	elif base:
	rval = prepend_base(base, rval)

base is usually passed though as an empty string "" so these conditions do not trigger.

Fixing returns this:

input = {
    "@context": {
        "a": {
            "@id": "http://example.org/a",
            "@type": "@id",
            "@context": {
                "@base": "http://example.org/",
            },
        },
    },
    "@id": "http://example.org/ex",
    "a": "a1",
}

expected = [
    {
        "@id": "http://example.org/ex",
        "http://example.org/a": [{"@id": "http://example.org/a1"}],
    }
]

jsonld.expand(input) == expected