Support 0 based sheet numbering

[lipkap]

Hi - i just came across a weired xlsx where the worksheet xml filenames use zero based numbering:

xl/worksheets/sheet0.xml

while what you 'normally' see is something like:

xl/worksheets/sheet1.xml
xl/worksheets/sheet2.xml
xl/worksheets/sheet3.xml

This pull request contains a v. trivial fix for this

[coveralls]

Coverage remained the same at 92.69% when pulling 4702b18 on goodylabs:master into 707ed15 on roo-rb:master.

[simonoff]

@lipkap can you add some test cases?

[lipkap]

I will try - The only example sheet i have contains sensitive data that i cannot really share publicly but when I edit and save the worksheets get renumbered :)

[simonoff]

Ok, good.

[stevendaniels]

#196 should fix this issue.

@lipkap Can you create a gist (sample gist) with that xlsx document so that I can confirm #196 fixes the issue?

Here are some instructions for creating such a gist.

1. Create a gist with code that creates the error.
2. Clone the gist repo locally, add a stripped down version of the offending spreadsheet to the gist repo, and push the gist's changes master.
3. Paste the gist url here.

[stevendaniels]

I'm closing this request. #196 already covers this request, too.