Add switch_default_only format+KST

Demonstrates possible issues like kaitai-io/kaitai_struct#595
and kaitai-io/kaitai_struct#415

formats/switch_default_only.ksy

@@ -0,0 +1,26 @@
+meta:
+  id: switch_default_only
+seq:
+  - id: opcode
+    type: s1
+  - id: byte
+    type:
+      switch-on: opcode
+      cases:
+        _: s1
+  - id: struct
+    type:
+      switch-on: opcode
+      cases:
+        _: data
+  - id: struct_sized
+    type:
+      switch-on: opcode
+      cases:
+        _: data
+    size: 4
+types:
+  data:
+    seq:
+      - id: value
+        size: 4

spec/ks/switch_default_only.kst

@@ -0,0 +1,11 @@
+id: switch_default_only
+data: switch_opcodes.bin
+asserts:
+  - actual: opcode
+    expected: 0x53
+  - actual: byte
+    expected: 0x66
+  - actual: struct.value
+    expected: '[0x6f, 0x6f, 0x62, 0x61]'
+  - actual: struct_sized.value
+    expected: '[0x72, 0x00, 0x49, 0x42]'