add Marshal/Unmarshal text (#2)

Co-authored-by: andrey1s <andrey@4devs.pro>
Reviewed-on: #2
Co-authored-by: andrey <andrey@4devs.io>
Co-committed-by: andrey <andrey@4devs.io>

mime_example_test.go

@@ -63,10 +63,11 @@ func ExampleMime_jsonMarshal() {
 		Mime: mime.TextMarkdown,
 	}
 
-	result, _ := json.Marshal(res)
+	result, err := json.Marshal(res)
 
-	fmt.Printf("%s", result)
+	fmt.Printf("%s\n%v", result, err)
 
 	// Output:
 	// {"data":"some text","mime":"text/markdown"}
+	// <nil>
 }