prevent ArgumentError: wrong number of arguments (0 for 1) in Roo::Base#...

...find by row number if header_line is not nil.

Roo::Base#find not works with row number as argument if header_line is set. 
I think this need to be fixed because header line is not nil as default:
https://github.com/Empact/roo/blob/master/lib/roo/base.rb#L61

Output from console:
1.9.2-p320 :010 > excel = Roo::Excel.new("/Users/file.xls")
1.9.2-p320 :017 >   excel.header_line
 => 1
1.9.2-p320 :018 > excel.find(1)
ArgumentError: wrong number of arguments (0 for 1)
from /Users/funkydrummer/.rvm/gems/ruby-1.9.2-p320/gems/roo-1.13.2/lib/roo/base.rb:263:in `row'
from /Users/funkydrummer/.rvm/gems/ruby-1.9.2-p320/gems/roo-1.13.2/lib/roo/base.rb:229:in `find'
from (irb):18
from /Users/funkydrummer/.rvm/rubies/ruby-1.9.2-p320/bin/irb:16:in `<main>'

This pull request fix this issue.
So, now:
excel.header_line = 5
excel.row(1) - return first row
excel.find(1) - return fifth row of spreadsheet