Simple image_tag edge-case fix

Currently image_tag throws an error generating the alt attribute for
images whose basename contains only periods.

Suggest simply casting possible nil to_s.

- options[:alt] ||= File.basename(options[:src],
'.*').split('.').first.capitalize
+ options[:alt] ||= File.basename(options[:src],
'.*').split('.').first.to_s.capitalize

Addressed in ticket 9018, patch at
http://dev.rubyonrails.org/attachment/ticket/9018/9018_image_tag_alt_attribute_for_files_whose_basename_contains_only_periods.diff