libSBOLj serializes Boolean annotations as strings

[jakebeal]

While annotations retain their types internally, when you send a Boolean annotation to SynBioHub, it comes back as a string. This appears to be due to an issue in libSBOLj serialization, which is not adding the appropriate rdf:datatype annotations.

For example:
<ns0:stub_object>true</ns0:stub_object>
should instead be:
<ns0:stub_object rdf:datatype="http://www.w3.org/2001/XMLSchema#boolean" >true</ns0:stub_object>

When this is resolved, tests should be added to see if this issue is also afflicting other libraries. Likewise, are there other places in SBOL where Booleans are used?

[cjmyers]

I'm not aware of any other booleans in SBOL other than annotations.

[jakebeal]

I thought we'd used them in a property somewhere, but I just went through and find it nowhere except on annotations, so it looks like that's the only place where this is a problem.