patch 9.0.1487: Content-type header for LSP channel not according to spec

Problem:    Content-type header for LSP channel not according to spec.
Solution:   Use "vscode-jsonrpc" instead of "vim-jsonrpc". (Yegappan
            Lakshmanan, closes #12295)
diff --git a/src/json.c b/src/json.c
index c1602c3..acf7ac5 100644
--- a/src/json.c
+++ b/src/json.c
@@ -103,9 +103,10 @@
     ga_append(&ga, NUL);
 
     ga_init2(&lspga, 1, 4000);
+    // Header according to LSP specification.
     vim_snprintf((char *)IObuff, IOSIZE,
 	    "Content-Length: %u\r\n"
-	    "Content-Type: application/vim-jsonrpc; charset=utf-8\r\n\r\n",
+	    "Content-Type: application/vscode-jsonrpc; charset=utf-8\r\n\r\n",
 	    ga.ga_len - 1);
     ga_concat(&lspga, IObuff);
     ga_concat_len(&lspga, ga.ga_data, ga.ga_len);