[ruby/stringio] Convert whence to Integer again in StringIO#seek

d3c5e6d compared whence with Fixnums directly, so a Float or an object with `to_int`, which IO#seek and the JRuby version accept, raised Errno::EINVAL.

https://github.com/ruby/stringio/commit/6ecc66a590

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ext/stringio/stringio.c

@@ -960,15 +960,14 @@ strio_seek(int argc, VALUE *argv, VALUE self)
     if (CLOSED(self)) {
 	rb_raise(rb_eIOError, "closed stream");
     }
-    switch (whence) {
-      case Qnil:
-      case INT2FIX(0):
+    switch (NIL_P(whence) ? 0 : NUM2LONG(whence)) {
+      case 0:
 	offset = 0;
 	break;
-      case INT2FIX(1):
+      case 1:
 	offset = ptr->pos;
 	break;
-      case INT2FIX(2):
+      case 2:
 	if (NIL_P(ptr->string)) {
 	    offset = 0;
 	}

test/stringio/test_stringio.rb

@@ -513,6 +513,8 @@ class TestStringIO < Test::Unit::TestCase
     assert_raise(Errno::EINVAL) { f.seek(-1) }
     f.seek(-1, 2)
     assert_equal("4", f.getc)
+    f.seek(-2, 2.0)
+    assert_equal("3", f.getc)
     assert_raise(Errno::EINVAL) { f.seek(1, 3) }
     f.close
     assert_raise(IOError) { f.seek(0) }