Fix "null" default not being set on schema (#296)

Fixes #295

schemars/src/_private.rs

@@ -161,7 +161,7 @@ pub mod metadata {
     add_metadata_fn!(add_deprecated, deprecated, bool);
     add_metadata_fn!(add_read_only, read_only, bool);
     add_metadata_fn!(add_write_only, write_only, bool);
-    add_metadata_fn!(add_default, default, Value);
+    add_metadata_fn!(add_default, default, Option<Value>);
 
     pub fn add_examples<I: IntoIterator<Item = Value>>(schema: Schema, examples: I) -> Schema {
         let mut schema_obj = schema.into_object();

schemars/tests/default.rs

@@ -30,6 +30,7 @@ where
 struct MyStruct {
     my_int: i32,
     my_bool: bool,
+    my_optional_string: Option<String>,
     #[serde(serialize_with = "custom_serialize")]
     my_struct2: MyStruct2,
     #[serde(

schemars/tests/expected/default.json

@@ -12,6 +12,13 @@
       "default": false,
       "type": "boolean"
     },
+    "my_optional_string": {
+      "default": null,
+      "type": [
+        "string",
+        "null"
+      ]
+    },
     "my_struct2": {
       "default": "i:0 b:false",
       "allOf": [