[ruby/stringio] Fix the result of `StringIO#truncate` so compatible with `File`

https://github.com/ruby/stringio/commit/16847fea32

ext/stringio/stringio.c

@@ -1668,7 +1668,7 @@ strio_truncate(VALUE self, VALUE len)
     if (plen < l) {
 	MEMZERO(RSTRING_PTR(string) + plen, char, l - plen);
     }
-    return len;
+    return INT2FIX(0);
 }
 
 /*

test/stringio/test_stringio.rb

@@ -39,11 +39,11 @@ class TestStringIO < Test::Unit::TestCase
   def test_truncate
     io = StringIO.new("")
     io.puts "abc"
-    io.truncate(0)
+    assert_equal(0, io.truncate(0))
     io.puts "def"
     assert_equal("\0\0\0\0def\n", io.string, "[ruby-dev:24190]")
     assert_raise(Errno::EINVAL) { io.truncate(-1) }
-    io.truncate(10)
+    assert_equal(0, io.truncate(10))
     assert_equal("\0\0\0\0def\n\0\0", io.string)
   end