Properly process paths/files with spaces in them.

Fixes the following (new) test:

    1) Roo::Spreadsheet.open with spaces in the filename loads the proper type
       Failure/Error: Roo::Spreadsheet.open(filename)
       URI::InvalidURIError:
         bad URI(is not URI?): path with spaces.xls
       # ./lib/roo/spreadsheet.rb:12:in `open'