Debug shared_string error.

This bug is reported by Qiong Peng and the patch is provided by Moo Yu.
Thank you, Qiong Peng, Moo Yu and Thierry Thelliez.
In detail, refer to
http://rubyforge.org/tracker/index.php?func=detail&aid=28007&group_id=678&atid=2677
http://dev.ywesee.com/wiki.php/Gem/Spreadsheet
Added Version to lib/spreadsheet.rb and Updated Manifest.
Updated History.txt, Rakefile and lib/spreadsheet.rb (this time
hopefully)
updated test/integration.rb thanks for the hint tomiacannondale@gmail.com
Updated History.txt
updated VERSION to 0.6.5.2
Updated README.txt
Updated README.txt
Updated README.txt to inlcude Link to graphical overview
Updated Rakefile so 'rake git:manifest' can be run.
encoding fix for ruby 1.9.2
encoding fix for ruby 1.9.2
updated lib/spreadsheet/excel/writer/biff8.rb according to Feedback from Alexandre Bini, also see the Changlog
Updated History.txt, README.txt and lib/spreadsheet.rb
Patch from Bjoern Andersson for Exploding on write when NaN is encountered
Added a test for the patch from Bjoern Andersson regarding NaN (Not a Number)
Updated History.txt and changed Version in lib/spreadsheet.rb
Add merged cells functionality
Add worksheet protection
updated the color code for orange to 0x0034 => :orange, thanks to the hint of Jonty