Fix spurious errors with networking tests.

crates/cargo-test-support/src/registry.rs

@@ -781,6 +781,7 @@ impl HttpServer {
             let buf = buf.get_mut();
             write!(buf, "HTTP/1.1 {}\r\n", response.code).unwrap();
             write!(buf, "Content-Length: {}\r\n", response.body.len()).unwrap();
+            write!(buf, "Connection: close\r\n").unwrap();
             for header in response.headers {
                 write!(buf, "{}\r\n", header).unwrap();
             }

tests/testsuite/publish.rs

@@ -1965,6 +1965,7 @@ Caused by:
   headers:
   <tab>HTTP/1.1 400
   <tab>Content-Length: 7
+  <tab>Connection: close
   <tab>
   body:
   go away