Update account.rb

if number is nil, you will set @bank_number to 0 (same for @bank_account_number)

2.0.0p247 :001 > number = nil
 => nil 
2.0.0p247 :002 > nr_str = "#{number}".gsub(/\s/,'')
 => "" 
2.0.0p247 :003 > @bank_number = nr_str.to_i
 => 0 
2.0.0p247 :004 >
added tests for argument validation of bank account number and bank number if these are nil
added test for not tested currency size exception
Merge pull request #14 from jethroo/patch-1

Don't allow nil for bank_number and bank_account_number