fix #514: fromxlsx with read_only crashes

[juarezr]

This PR fixes a problem reported on #514

Changes

1. Added test for fromxlsx(read_only=True)
2. Fixed a bug in fromxlsx with read_only=True

Checklist

Checklist for for pull requests including new code and/or changes to existing code...

• Code
  • Includes unit tests
  • New functions have docstrings with examples that can be run with doctest
  • New functions are included in API docs
  • Docstrings include notes for any changes to API or behaviour
  • All changes documented in docs/changes.rst
• Testing
  • (Optional) Tested local against remote servers
  • Travis CI passes (unit tests run under Linux)
  • AppVeyor CI passes (unit tests run under Windows)
  • Unit test coverage has not decreased (see Coveralls)
• Changes
  • Ready to review
  • Ready to merge

[coveralls]

Coverage decreased (-0.9%) to 91.071% when pulling c4aa8a4 on juarezr:fix_xlsx_readonly into feeb3be on petl-developers:master.