Fixed header row type mixup.

[hannesl]

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.

[Empact]

Hey @hannesl I suspect this is supposed to work a bit differently. If you read the #each documentation it says the values need to be found in the sheet. If they aren't, then the row_with method improperly returns the return value of #each - in this case 1. I just made a commit to fail informatively - how is that?

[stevendaniels]

These changes were previously been merged into master.