Added fix for calling 'close' on a StringIO-backed zip file, and specs

[pnomolos]

Right now if you call Zip::File#close on a zipfile that's been opened with a StringIO you get an exception in on_success_replace. This fixes this since calling .close here should be a no-op anyway as there's no on-disk file to replace.

[coveralls]

Coverage increased (+3.7%) to 96.486% when pulling cadd48f on pnomolos:pnomolos/fix_stringio into 2f80da6 on rubyzip:master.

[coveralls]

Coverage increased (+3.8%) to 96.52% when pulling 792266d on pnomolos:pnomolos/fix_stringio into 2f80da6 on rubyzip:master.

[coveralls]

Coverage increased (+3.8%) to 96.52% when pulling 792266d on pnomolos:pnomolos/fix_stringio into 2f80da6 on rubyzip:master.