Fix argument handling in IO::Buffer#each_byte (ruby#15309)

The method incorrectly ignored its first argument and treated the second argument as offset and the third as count.
This change makes the first argument be treated as offset and the second as count.

Also fix incorrect block parameter in comments.

Author: tmtm

io_buffer.c

@@ -2231,7 +2231,7 @@ io_buffer_values(int argc, VALUE *argv, VALUE self)
 
 /*
  *  call-seq:
- *    each_byte([offset, [count]]) {|offset, byte| ...} -> self
+ *    each_byte([offset, [count]]) {|byte| ...} -> self
  *    each_byte([offset, [count]]) -> enumerator
  *
  *  Iterates over the buffer, yielding each byte starting from +offset+.
@@ -2255,7 +2255,7 @@ io_buffer_each_byte(int argc, VALUE *argv, VALUE self)
     rb_io_buffer_get_bytes_for_reading(self, &base, &size);
 
     size_t offset, count;
-    io_buffer_extract_offset_count(RB_IO_BUFFER_DATA_TYPE_U8, size, argc-1, argv+1, &offset, &count);
+    io_buffer_extract_offset_count(RB_IO_BUFFER_DATA_TYPE_U8, size, argc, argv, &offset, &count);
 
     for (size_t i = 0; i < count; i++) {
         unsigned char *value = (unsigned char *)base + i + offset;

test/ruby/test_io_buffer.rb

@@ -473,6 +473,7 @@ def test_each_byte
     buffer = IO::Buffer.for(string)
 
     assert_equal string.bytes, buffer.each_byte.to_a
+    assert_equal string.bytes[3, 5], buffer.each_byte(3, 5).to_a
   end
 
   def test_zero_length_each_byte