Mark OSX::NSData#dataWithRubyString() will be deprecated

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2611 f4f36d40-9719-0410-9868-dde021a23eda
Replace NSData#dataWithString() to NSString#dataUsingEncoding()

NSData#dataUsingEncoding() will be deprecated.

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2612 f4f36d40-9719-0410-9868-dde021a23eda
Add an assertion for NSData#length contains not ASCII data

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2613 f4f36d40-9719-0410-9868-dde021a23eda