Adds failing test for numbered sheet names bug

Code makes assumption from filenames that if sheet2.xml exists,
sheet1.xml will exist.  Test case attempts to read all sheets from a
file that only has sheet2.xml.
Fixes bug with missing sheetN.xml files in XLSX

Workaround is to omit nil values from the array which was inserted into
via numeric indices.
Merge pull request #58 from benizi/bug-numbered-sheet-names

Bug with numbered sheet filenames