String values should pass their content-type correctly on http requests with a body (e.g. http post) (#13731)

# Description
The content-type was not being handled appropriately when sending
requests with a string value.

# User-Facing Changes
- Passing a string value through a pipeline with a content-type set as
metadata is handled correctly
- Passing a string value as a parameter with a content-type set as a
flag is handled correctly.

crates/nu-command/tests/commands/network/http/post.rs

@@ -115,6 +115,24 @@ fn http_post_json_is_success() {
     assert!(actual.out.is_empty())
 }
 
+#[test]
+fn http_post_json_string_is_success() {
+    let mut server = Server::new();
+
+    let mock = server
+        .mock("POST", "/")
+        .match_body(r#"{"foo":"bar"}"#)
+        .create();
+
+    let actual = nu!(format!(
+        r#"http post -t 'application/json' {url} '{{"foo":"bar"}}'"#,
+        url = server.url()
+    ));
+
+    mock.assert();
+    assert!(actual.out.is_empty())
+}
+
 #[test]
 fn http_post_json_list_is_success() {
     let mut server = Server::new();
@@ -149,7 +167,7 @@ fn http_post_json_int_is_success() {
 }
 
 #[test]
-fn http_post_json_string_is_success() {
+fn http_post_json_raw_string_is_success() {
     let mut server = Server::new();
 
     let mock = server.mock("POST", "/").match_body(r#""test""#).create();