StringIO#seek should work even if String is frozen

[headius]

A regression from #116 only in the JRuby extension.

TestCSVInterfaceReadWrite::DifferentOFS#test_filter:
IOError: not modifiable string
    org/jruby/ext/stringio/StringIO.java:1350:in `seek'
    /Users/headius/work/jruby/lib/ruby/stdlib/csv/parser.rb:683:in `resolve_row_separator'
    /Users/headius/work/jruby/lib/ruby/stdlib/csv/parser.rb:576:in `prepare_separators'
    /Users/headius/work/jruby/lib/ruby/stdlib/csv/parser.rb:442:in `prepare'
    /Users/headius/work/jruby/lib/ruby/stdlib/csv/parser.rb:329:in `initialize'
    org/jruby/RubyClass.java:936:in `new'
    /Users/headius/work/jruby/lib/ruby/stdlib/csv.rb:2824:in `parser'
    /Users/headius/work/jruby/lib/ruby/stdlib/csv.rb:2833:in `parser_enumerator'
    /Users/headius/work/jruby/lib/ruby/stdlib/csv.rb:2554:in `each'
    /Users/headius/work/jruby/lib/ruby/stdlib/csv.rb:1239:in `filter'
    /Users/headius/work/jruby/test/mri/csv/interface/test_read_write.rb:14:in `test_filter'

This CVS test creates a StringIO using a frozen String and later calls StringIO#seek. Changes in #116 incorrectly checked if the String is frozen and raised an error.

The StringIO#seek definition in the JRuby extension shall be modified to allow frozen Strings.