Add support for reading Zip64 size metadata

RubyZip can already correctly read files that are stored in the Zip64 format, but without this patch, rubyzip mistakenly believes that the files are of size 0xFFFFFFFF (4294967295). 

I've been seeing more and more zips that do not actually have large files inside, but still have the size data stored in the 64-bit format. 

This is my first attempt to submit a patch to this project - can you give me some advice on what sort of testing is recommended for a commit like this?