Add Support for HTTPHeaders

[0xLeif]

Example Use Case

When needing to set headers for a request.

extension URLRequest {
    mutating func dataTaskPublish(method: String = "GET", withBody body: Data? = nil) -> URLSession.DataTaskPublisher {
        let headers = [
            "Content-Type": "application/json; charset=utf-8",
            "cache-control": "no-cache",
        ]
        
        httpMethod = method
        allHTTPHeaderFields = headers
        if let body = body {
            httpBody = body
        }
        
        let session = URLSession.shared
        return session.dataTaskPublisher(for: self)
    }
}