Bug fix : Worksheet#write_url should not change internal url string.
Bug fix : typo in Worksheet#write_url_external
Bug fix : Worksheet#write_url_external and #write_url_external_net should not change external url string.
Bug fix : Worksheet#write_url_internal
add examples/hyperlink2.rb
add v0.6.19 comment.
set version to v0.6.19.