Fix optional UserAgent

Added an example

example_test.go

@@ -28,3 +28,33 @@ func ExampleSearch() {
 	// Output: Australia Wide First Aid (https://www.australiawidefirstaid.com.au/) found in the serp
 
 }
+
+/*
+Example of how to set the useragent
+*/
+func ExampleUserAgent() {
+
+	// whatismybrowser.com maintains a database of UserAgents
+	// https://www.whatismybrowser.com/guides/the-latest-user-agent/chrome
+
+	opt := SearchOptions{
+		CountryCode: "au",
+		UserAgent:   "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/113.0.0.0 Safari/537.36",
+	}
+
+	serp, err := Search(context.Background(), "First Aid Course Australia Wide First Aid", opt)
+
+	if err != nil {
+		fmt.Print(err.Error())
+	}
+
+	for _, resault := range serp {
+		if strings.Contains(resault.URL, "australiawidefirstaid.com.au") {
+			fmt.Println("Australia Wide First Aid (https://www.australiawidefirstaid.com.au/) found in the serp")
+			break
+		}
+	}
+
+	// Output: Australia Wide First Aid (https://www.australiawidefirstaid.com.au/) found in the serp
+
+}

search.go

@@ -31,6 +31,7 @@ type Result struct {
 }
 
 const stdGoogleBase = "https://www.google."
+const defaultAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/113.0.0.0 Safari/537.36"
 
 // GoogleDomains represents localized Google homepages. The 2 letter country code is based on ISO 3166-1 alpha-2.
 //
@@ -284,7 +285,8 @@ func Search(ctx context.Context, searchTerm string, opts ...SearchOptions) ([]Re
 	}
 
 	if opts[0].UserAgent == "" {
-		c.UserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/113.0.0.0 Safari/537.36"
+		c.UserAgent = defaultAgent
+	} else {
 		c.UserAgent = opts[0].UserAgent
 	}