Quick-fix blob.slice() contentType argument handling
Quick-fix blob.slice() contentType argument handling

This algorithm still needs an overhaul, see #1681, but for now this will get the newly-imported web platform tests passing.

snuggs/jsdom,Zirro/jsdom,tmpvar/jsdom,mzgol/jsdom,tmpvar/jsdom,mzgol/jsdom,snuggs/jsdom,Zirro/jsdom,mzgol/jsdom