fix: improve Host header handling in addHeaders function

sling.go

@@ -329,10 +329,17 @@ func addQueryStructs(reqURL *url.URL, queryStructs []interface{}) error {
 
 // addHeaders adds the key, value pairs from the given http.Header to the
 // request. Values for existing keys are appended to the keys values.
+// If the key is "Host", the Host header is set to the first value.
 func addHeaders(req *http.Request, header http.Header) {
 	for key, values := range header {
-		for _, value := range values {
-			req.Header.Add(key, value)
+		if key == "Host" && len(values) > 0 {
+			// For Host header, only set the Host field directly
+			req.Host = values[0]
+		} else {
+			// For all other headers, add them to the request header
+			for _, value := range values {
+				req.Header.Add(key, value)
+			}
 		}
 	}
 }

sling_test.go

@@ -1010,3 +1010,40 @@ func assertPostForm(t *testing.T, expected map[string]string, req *http.Request)
 		t.Errorf("expected parameters %v, got %v", expected, req.PostForm)
 	}
 }
+
+func TestRequest_hostHeader(t *testing.T) {
+	cases := []struct {
+		sling        *Sling
+		expectedHost string
+	}{
+		// Test setting Host header
+		{New().Add("Host", "example.com"), "example.com"},
+		// Test setting Host header with port
+		{New().Add("Host", "example.com:8080"), "example.com:8080"},
+		// Test empty Host header (should not panic)
+		{New().Add("Host", ""), ""},
+		// Test multiple Host headers (should use first value)
+		{New().Add("Host", "first.com").Add("Host", "second.com"), "first.com"},
+		// Test Host header with other headers
+		{New().Add("Content-Type", "application/json").Add("Host", "example.com"), "example.com"},
+		// Test Host header case insensitivity
+		{New().Add("host", "example.com"), "example.com"},
+		{New().Add("HOST", "example.com"), "example.com"},
+	}
+
+	for _, c := range cases {
+		req, err := c.sling.Request()
+		if err != nil {
+			t.Errorf("unexpected error: %v", err)
+			continue
+		}
+		if req.Host != c.expectedHost {
+			t.Errorf("expected Host %q, got %q", c.expectedHost, req.Host)
+		}
+		// Verify that Host header is not duplicated in Header map
+		if len(req.Header["Host"]) > 0 {
+			t.Errorf("Host header should not be in Header map, got %v", req.Header["Host"])
+		}
+	}
+}
+