Update Host header only when the user intentionally provide a value

[TharmiganK]

Description:

With this fix: #6133 the host header is not overwritten by the default host name inferred from the client url. But there are use cases like passthrough where we just send back the request. In those scenarios we should use the host name from the client url unless the user overwrites it.

Describe your problem(s)

import ballerina/http;
import ballerina/io;

service /api on new http:Listener(9090) {

    resource function 'default [string... path](@http:Header string host) returns string {
        io:println("Host header: ", host); // This will be: Host header: localhost:9091
        return "Hello, World!";
    }
}

final http:Client clientEP = check new ("localhost:9090");

service /api on new http:Listener(9091) {

    resource function 'default [string... path](http:Request req) returns http:Response|error {
        return clientEP->execute(req.method, req.rawPath, req);
    }
}

$ curl -v http://localhost:9090/api/test

*   Trying [::1]:9091...
* Connected to localhost (::1) port 9091
> GET /api/test HTTP/1.1
> Host: localhost:9091
> User-Agent: curl/8.4.0
> Accept: */*
> 
< HTTP/1.1 200 OK
< content-type: text/plain
< date: Thu, 7 Mar 2024 15:00:45 +0530
< server: ballerina
< content-length: 13
< 
* Connection #0 to host localhost left intact
Hello, World!

Describe your solution(s)

Use the default host from the client url when the user not provided a value.

[TharmiganK]

Fixed with the above PRs