Nokogiri hangs with large files

Hi Empact.

Some (long) time ago you pull into your fork one modification I made for large files that were hanging with nokogiri, and you made a little update regarding namespaces to it (d4a8e6f779e38c113967f944869dd45f498e4388).

Today I decide to change my version and use your fork that looks a lot more updated (congratulations!). But when trying to process the same large files, the process hangs again.

So I decide to put my original lines in your version (in both methods, read_cells and read_shared_strings), and the code start to work again.

Unfortunately I can't upload the files because has private data, but is a file with 300.000 rows and 3 columns. In each row; the first cell has an average of 30 chars, the second one 11 chars, and in the last one, only 1 char.

Is not very clear to me why is this happening, and although I already update my fork with your changes and my fix, but it would be great if you can consider that modification cause i prefer to use your version that is by far more complete :)