Avoid frozen error in _toUTF8 (Fixes #83)

[jeremyevans]

This happens when attempting to set an encoding on the string. If the string is frozen, use +@ to duplicate it (so that chilled strings are handled correctly).

This happens at least in sample/figmemo_sample.rb on Tcl/Tk 9.0.

Backtrace:

lib/tk.rb:2813:in 'TclTkIp#_toUTF8': can't modify frozen String: "10c" (FrozenError)
        from lib/tk.rb:2813:in 'TclTkIp#_toUTF8'
        from lib/tk.rb:676:in 'TkComm#_toUTF8'
        from lib/tk.rb:1962:in 'TkUtil#_conv_args'
        from lib/tk.rb:1962:in 'TkCore#_tk_call_core'
        from lib/tk.rb:1990:in 'TkCore#tk_call'
        from lib/tk.rb:3920:in 'TkConfigMethod#__configure_core'
        from lib/tk.rb:3943:in 'TkConfigMethod#configure'
        from lib/tk/composite.rb:329:in 'TkComposite#configure'
        from lib/tk.rb:4841:in 'TkObject#method_missing'
        from sample/figmemo_sample.rb:96:in 'PhotoCanvas#initialize'
        from sample/figmemo_sample.rb:415:in 'TkKernel.new'
        from sample/figmemo_sample.rb:415:in '<main>'

[mtasaka]

Okay, this is working for me, thank you.