Another followup from a while ago,

        * srcprop.c (scm_source_properties): Return plist from hash if it's a
        list set by source-properties! rather than an SRCPROPS object,

        * tests/srcprop.test: New file.
        * Makefile.am (SCM_TESTS): Add it.

Maybe set-source-properties! should store an srcprop object like the
comments say, but this change at least lets source-properties and
set-source-properties! cooperate (currently what's set can't be read).

This problem is also present in the 1.6 branch, if it's important
enough to be worth addressing there.