feat: make fd0's priority higher than fd1 when same tag being used by…

… both

BREAKING CHANGE: All top-level APIs will seek ifd0 tags first instead of ifd1, to retrieve tags from ifd1, use `Exif::Data#[]`, for example:

    ```ruby
    # to retrieve image width of ifd0
    data.image_width
    data[:ifd0][:image_width]
    # to retrieve image width of ifd1
    data[:ifd1][:image_width]
    ```

ext/exif/data.c

@@ -96,7 +96,9 @@ static void each_entry(ExifEntry *entry, void *self_ptr){
     ID2SYM(rb_intern(ivar_name + 1)),
     value
   );
-  rb_iv_set(*(VALUE*)self_ptr, ivar_name, value);
+  if(TYPE(rb_ivar_defined(*(VALUE*)self_ptr, rb_intern(ivar_name))) == T_FALSE){
+    rb_iv_set(*(VALUE*)self_ptr, ivar_name, value);
+  }
 }
 
 static VALUE exif_entry_to_rb_value(ExifEntry *entry){

test/test_exif.rb

@@ -162,8 +162,8 @@ def test_ascii
     end
 
     def test_rational
-      # assert_equal Rational(72, 1), data.x_resolution
-      # assert_equal Rational(72, 1), data.y_resolution
+      assert_equal Rational(300, 1), data.x_resolution
+      assert_equal Rational(300, 1), data.y_resolution
       assert_equal Rational(1, 125), data.exposure_time
       assert_equal Rational(8, 1), data.fnumber
       assert_equal Rational(870723, 125000), data.shutter_speed_value