trim whitespace before adding identifier prefixes

some sources have erroneous whitespace. when we add prefixes, if the whitespace isn't trimmed, we will have accidental whitespace in the identifier. for example:

http://neuinfo.org/servicesv1/v1/federation/data/nlx_151835-1.json?exportType=data&q=HP:0000238+OMIM:610829&includeSubclasses=true&limit=10

so, we need to make sure to add a btrim() to any item that we prefix with an idspace.