Handling BigDecimals

Based on the comments on https://groups.google.com/forum/?fromgroups#!topic/rubyspreadsheet/2sKXgIFPrKs and my own experience, we know that the gem doesn't handle BigDecimal precision.

Looking around, I found the fix introduced by the user 'valeriusjames' and it seems to be working fine. I don't know why he hasn't sent request, but I thing that is something to be considered...