Allow at-rules in keyframe blocks

See sass/sass#3859
See sass/sass#3861

spec/css/keyframes.hrx

@@ -251,45 +251,45 @@ $a: b;
 
 <===>
 ================================================================================
-<===> error/in_keyframe_block/style_rule/input.scss
+<===> in_keyframe_block/unknown_at_rule/input.scss
 @keyframes a {
-  to {to {c: d}}
+  to {@b}
 }
 
-<===> error/in_keyframe_block/style_rule/error
-Error: Style rules may not be used within keyframe blocks.
-  ,
-2 |   to {to {c: d}}
-  |       ^^^^^^^^^
-  '
-  input.scss 2:7  root stylesheet
+<===> in_keyframe_block/unknown_at_rule/output.css
+@keyframes a {
+  to {
+    @b;
+  }
+}
 
 <===>
 ================================================================================
-<===> error/in_keyframe_block/unknown_at_rule/input.scss
+<===> in_keyframe_block/known_at_rule/input.scss
 @keyframes a {
-  to {@b}
+  to {@media screen {b: c}}
 }
 
-<===> error/in_keyframe_block/unknown_at_rule/error
-Error: At-rules may not be used within keyframe blocks.
-  ,
-2 |   to {@b}
-  |       ^^
-  '
-  input.scss 2:7  root stylesheet
+<===> in_keyframe_block/unknown_at_rule/output.css
+@keyframes a {
+  to {
+    @media screen {
+      b: c;
+    }
+  }
+}
 
 <===>
 ================================================================================
-<===> error/in_keyframe_block/known_at_rule/input.scss
+<===> error/in_keyframe_block/style_rule/input.scss
 @keyframes a {
-  to {@media screen {b: c}}
+  to {to {c: d}}
 }
 
-<===> error/in_keyframe_block/known_at_rule/error
-Error: At-rules may not be used within keyframe blocks.
+<===> error/in_keyframe_block/style_rule/error
+Error: Style rules may not be used within keyframe blocks.
   ,
-2 |   to {@media screen {b: c}}
-  |       ^^^^^^^^^^^^^^^^^^^^
+2 |   to {to {c: d}}
+  |       ^^^^^^^^^
   '
   input.scss 2:7  root stylesheet