Fix flattening of serde_json::Value

It should behave similarly to flattening a `Map<String,Value>` in that it allows any properties

schemars/tests/expected/flattened_value.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "FlattenValue",
+  "type": "object",
+  "properties": {
+    "flag": {
+      "type": "boolean"
+    }
+  },
+  "required": [
+    "flag"
+  ],
+  "additionalProperties": true
+}

schemars/tests/flatten.rs

@@ -1,5 +1,7 @@
 mod util;
 use schemars::JsonSchema;
+use serde_json::Value;
+use std::collections::BTreeMap;
 use util::*;
 
 #[allow(dead_code)]
@@ -53,5 +55,34 @@ fn test_flat_schema() -> TestResult {
 
 #[test]
 fn test_flattened_schema() -> TestResult {
+    // intentionally using the same file as test_flat_schema, as the schema should be identical
     test_default_generated_schema::<Deep1>("flatten")
 }
+
+#[allow(dead_code)]
+#[derive(JsonSchema)]
+struct FlattenValue {
+    flag: bool,
+    #[serde(flatten)]
+    value: Value,
+}
+
+#[allow(dead_code)]
+#[derive(JsonSchema)]
+#[schemars(rename = "FlattenValue")]
+struct FlattenMap {
+    flag: bool,
+    #[serde(flatten)]
+    value: BTreeMap<String, Value>,
+}
+
+#[test]
+fn test_flattened_value() -> TestResult {
+    test_default_generated_schema::<FlattenValue>("flattened_value")
+}
+
+#[test]
+fn test_flattened_map() -> TestResult {
+    // intentionally using the same file as test_flattened_value, as the schema should be identical
+    test_default_generated_schema::<FlattenMap>("flattened_value")
+}