change from 400 to 415

doc/PROTOCOL-HTTP2.md

@@ -61,7 +61,7 @@ the form shown above.
 
 If **Timeout** is omitted a server should assume an infinite timeout. Client implementations are free to send a default minimum timeout based on their deployment requirements.
 
-If **Content-Type** does not begin with "application/grpc", gRPC servers SHOULD respond with HTTP status of 400 (Bad Request).  This will prevent other HTTP/2 clients from interpreting a gRPC error response, which uses status 200 (OK), as successful.
+If **Content-Type** does not begin with "application/grpc", gRPC servers SHOULD respond with HTTP status of 415 (Unsupported Media Type).  This will prevent other HTTP/2 clients from interpreting a gRPC error response, which uses status 200 (OK), as successful.
 
 **Custom-Metadata** is an arbitrary set of key-value pairs defined by the application layer. Header names starting with "grpc-" but not listed here are reserved for future GRPC use and should not be used by applications as **Custom-Metadata**.