rpc: add SetHeader method to Client (#21392)

Resolves #20163

Co-authored-by: Felix Lange <fjl@twurst.com>

rpc/client_test.go

@@ -26,6 +26,7 @@ import (
 	"os"
 	"reflect"
 	"runtime"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -429,6 +430,42 @@ func TestClientNotificationStorm(t *testing.T) {
 	doTest(23000, true)
 }
 
+func TestClientSetHeader(t *testing.T) {
+	var gotHeader bool
+	srv := newTestServer()
+	httpsrv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Header.Get("test") == "ok" {
+			gotHeader = true
+		}
+		srv.ServeHTTP(w, r)
+	}))
+	defer httpsrv.Close()
+	defer srv.Stop()
+
+	client, err := Dial(httpsrv.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	client.SetHeader("test", "ok")
+	if _, err := client.SupportedModules(); err != nil {
+		t.Fatal(err)
+	}
+	if !gotHeader {
+		t.Fatal("client did not set custom header")
+	}
+
+	// Check that Content-Type can be replaced.
+	client.SetHeader("content-type", "application/x-garbage")
+	_, err = client.SupportedModules()
+	if err == nil {
+		t.Fatal("no error for invalid content-type header")
+	} else if !strings.Contains(err.Error(), "Unsupported Media Type") {
+		t.Fatalf("error is not related to content-type: %q", err)
+	}
+}
+
 func TestClientHTTP(t *testing.T) {
 	server := newTestServer()
 	defer server.Stop()