fix incorrect deserialization of some legacy formats

```
In at least some cases, the legacy format includes an additional field that needs to be considered.

# ruby 2.6.10p210 (2022-04-12 revision 67958) [universal.x86_64-darwin22]

require "ostruct"
require "yaml"
os = OpenStruct.new
os.a = 'b'
YAML.dump(os)

# => "--- !ruby/object:OpenStruct\ntable:\n  🅰️ b\nmodifiable: true\n"
```

Author: atogata

lib/ostruct.rb

@@ -435,9 +435,10 @@ def encode_with(coder) # :nodoc:
     @table.each_pair do |key, value|
       coder[key.to_s] = value
     end
-    if @table.size == 1 && @table.key?(:table) # support for legacy format
-      # in the very unlikely case of a single entry called 'table'
-      coder['legacy_support!'] = true # add a bogus second entry
+
+    # ensure data isn't corrupted on reload if it looks like the legacy format
+    if @table.size == 1 && @table.key?(:table) || @table.size == 2 && [:modifiable, :table].all? { |x| @table.key?(x) }
+      coder['legacy_support!'] = true # add a bogus entry to change the size
     end
   end
 
@@ -446,12 +447,12 @@ def encode_with(coder) # :nodoc:
   #
   def init_with(coder) # :nodoc:
     h = coder.map
-    if h.size == 1 # support for legacy format
-      key, val = h.first
-      if key == 'table'
-        h = val
-      end
+
+    # support for legacy format
+    if h.size == 1 && h.key?('table') || h.size == 2 && %w[modifiable table].all? { |x| h.key?(x) }
+      h = h['table']
     end
+
     update_to_values!(h)
   end
 

test/ostruct/test_ostruct.rb

@@ -390,6 +390,15 @@ def test_legacy_yaml
     assert_equal({foo: 42}, YAML.safe_load(YAML.dump(o), permitted_classes: [Symbol, OpenStruct]).table)
   end if RUBY_VERSION >= '2.6'
 
+  def test_legacy_yaml_2
+    s = "--- !ruby/object:OpenStruct\ntable:\n  :foo: 42\nmodifiable: true\n"
+    o = YAML.safe_load(s, permitted_classes: [Symbol, OpenStruct])
+    assert_equal(42, o.foo)
+
+    o = OpenStruct.new(table: {foo: 42})
+    assert_equal({foo: 42}, YAML.safe_load(YAML.dump(o), permitted_classes: [Symbol, OpenStruct]).table)
+  end if RUBY_VERSION >= '2.6'
+
   def test_yaml
     h = {name: "John Smith", age: 70, pension: 300.42}
     yaml = "--- !ruby/object:OpenStruct\nname: John Smith\nage: 70\npension: 300.42\n"