BibCirculation: get_book_cover quick fix

• Backports the get_book_cover fix from the master branch (see commit bb38915f3ac3ac4cdd252a995bd4caed77a9c3bb) into the maint-1.1 branch to prevent an "HTTPError: HTTP Error 400: Bad Request" from urllib2 when accessing the 'Item details' page which originated from the BIBCIRCULATION_OPENER.open call in the get_book_cover(isbn) function. (closes #2578) (PR #2653)

• NOTE The concerned book cover code bits in maint-1.1 and master are now identical, though the code could (and should!) be improved.

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>