Test and restore behavior around to_json changing depth

When serializing an Array, and one of the elements of the Array requires
calling `to_json`, if the depth is changed, it will be used for the next
entries, which wasn't the case before
5abd434, and is not the case with
TruffleRuby and JRuby.

Additionally, with TruffleRuby and JRuby the state's depth after the
`to_json` call is used to close the Array, which isn't the case with
CRuby.

Author: etiennebarrie

ext/json/ext/generator/generator.c

@@ -1294,6 +1294,8 @@ static void generate_json_fallback(FBuffer *buffer, struct generate_json_data *d
     VALUE tmp;
     if (rb_respond_to(obj, i_to_json)) {
         tmp = rb_funcall(obj, i_to_json, 1, vstate_get(data));
+        GET_STATE(data->vstate);
+        data->depth = state->depth;
         Check_Type(tmp, T_STRING);
         fbuffer_append_str(buffer, tmp);
     } else {

test/json/json_generator_test.rb

@@ -321,6 +321,30 @@ def test_allow_nan
     end
   end
 
+  def test_depth_bad_to_json
+    obj = Object.new
+    def obj.to_json(state)
+      state.depth += 1
+      "{#{state.object_nl}"\
+        "#{state.indent * state.depth}\"foo\":#{state.space}1#{state.object_nl}"\
+        "#{state.indent * (state.depth - 1)}}"
+    end
+    indent = "  " * 2 if RUBY_ENGINE != "ruby"
+    assert_equal <<~JSON.chomp, JSON.pretty_generate([obj] * 2)
+      [
+        {
+          "foo": 1
+        },
+        {
+            "foo": 1
+          }
+      #{indent}]
+    JSON
+    state = JSON::State.new(object_nl: "\n", array_nl: "\n", space: " ", indent: "  ")
+    state.generate(obj)
+    assert_equal 1, state.depth
+  end
+
   def test_depth
     pretty = { object_nl: "\n", array_nl: "\n", space: " ", indent: "  " }
     state = JSON.state.new(**pretty)