Fix test_upto fails on ruby-2.x

Range#each yields String when its element responds to :to_str
on ruby-2.0 or later.
see range_each() in range.c of ruby source code.

    str1 = OSX::NSString.stringWithString('A')
    str2 = OSX::NSString.stringWithString('Z')
    (str1..str2).each do |s|
	# `s' is OSX::NSString on ruby-1.8
	# `s' becomes String on ruby-2.0 or later
        :
    end

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2552 f4f36d40-9719-0410-9868-dde021a23eda
Fix OSX::NSString#slice!(num) not delete by given position on ruby-2.x.

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2553 f4f36d40-9719-0410-9868-dde021a23eda