avoid clobbering user-agent specified in headers
avoid clobbering user-agent specified in headers

fgregg/scrapelib,sunlightlabs/scrapelib,h4ck3rm1k3/scrapelib,dvogel/scrapelib,arowla/scrapelib,olberger/scrapelib,fgregg/scrapelib,h4ck3rm1k3/scrapelib,olberger/scrapelib,JoeGermuska/scrapelib,dvogel/scrapelib,pombredanne/scrapelib,arowla/scrapelib,JoeGermuska/scrapelib,jamesturk/scrapelib