[Rust Server] Improve RFC 13341 compliance for multipart/related (#19355)

* [Rust Server] Improve RFC 13341 compliance for multipart/related

* Update samples

Author: richardwhiuk

modules/openapi-generator/src/main/resources/rust-server/generate-multipart-related.mustache

@@ -30,7 +30,10 @@
 {{#-last}}
 
         // Write the body into a vec.
-        let mut body: Vec<u8> = vec![];
+        // RFC 13341 Section 7.2.1 suggests that the body should begin with a
+        // CRLF prior to the first boundary. The mime_multipart library doesn't
+        // do this, so we do it instead.
+        let mut body: Vec<u8> = vec![b'\r', b'\n'];
         write_multipart(&mut body, &boundary, &body_parts)
             .expect("Failed to write multipart body");
 {{/-last}}

samples/server/petstore/rust-server/output/multipart-v3/src/client/mod.rs

@@ -469,7 +469,10 @@ impl<S, C> Api<C> for Client<S, C> where
         }
 
         // Write the body into a vec.
-        let mut body: Vec<u8> = vec![];
+        // RFC 13341 Section 7.2.1 suggests that the body should begin with a
+        // CRLF prior to the first boundary. The mime_multipart library doesn't
+        // do this, so we do it instead.
+        let mut body: Vec<u8> = vec![b'\r', b'\n'];
         write_multipart(&mut body, &boundary, &body_parts)
             .expect("Failed to write multipart body");
 
@@ -741,7 +744,10 @@ impl<S, C> Api<C> for Client<S, C> where
         }
 
         // Write the body into a vec.
-        let mut body: Vec<u8> = vec![];
+        // RFC 13341 Section 7.2.1 suggests that the body should begin with a
+        // CRLF prior to the first boundary. The mime_multipart library doesn't
+        // do this, so we do it instead.
+        let mut body: Vec<u8> = vec![b'\r', b'\n'];
         write_multipart(&mut body, &boundary, &body_parts)
             .expect("Failed to write multipart body");