Fixed header row type mixup.

When using sheet.parse() with a columns hash, I got the following error:

    lib/roo/base.rb:588:in `set_headers': undefined method `map' for 1:Fixnum (NoMethodError)

Example code:

    xls = Roo::Spreadsheet.open filepath
    columns = {
      this: "This",
      that: "That"
    }
    rows = xls.sheet(0).parse columns <-- BOOM

Unless I'm using things incorrectly, this seems like a bug.