Fix crash when loading databook from XLS

[jpvanhal]

xlrd.open_workbook() function expects the file_contents argument to be a string.

Fixes #522

[codecov[bot]]

Codecov Report

Merging #563 (a2bc2ef) into master (5431834) will increase coverage by 0.36%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #563      +/-   ##
==========================================
+ Coverage   91.74%   92.11%   +0.36%     
==========================================
  Files          28       28              
  Lines        2823     2827       +4     
==========================================
+ Hits         2590     2604      +14     
+ Misses        233      223      -10     

Files Changed	Coverage Δ
src/tablib/formats/_xls.py	95.74% <100.00%> (+10.63%)	:arrow_up:
tests/test_tablib.py	98.79% <100.00%> (+<0.01%)	:arrow_up:

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