Adds handling of case where fields/bsall tag is absent from surveyor xml. In this case, fall back to fields/origin if its value is WRLC Digital Collections. Otherwise, populate holding_institution_display with a clean blank value rather than leaving it unset.  Fixes #208
Merge pull request #209 from gwu-libraries/t208-holding-inst-display-err

Adds handling of case where fields/bsall tag is absent from surveyor xml...