docs(example): add headers validation, minor README improvements

README.md

@@ -96,7 +96,7 @@ ValidationRule.query(
 ),
 ```
 
-##### 🟣 Request Headers Validation
+##### 🟢 Request Headers Validation
 
 ```dart
 ValidationRule.headers(

example/request.http

@@ -9,6 +9,17 @@ Content-Type: application/json
   "gender": "male"
 }
 
+### POST /person with invalid Content-Type header
+POST http://localhost:8080/person
+Content-Type: application/xml
+
+{
+  "username": "thecodexhub",
+  "age": 24,
+  "email": "[email protected]",
+  "gender": "male"
+}
+
 ### POST /person with no username and age field
 POST http://localhost:8080/person
 Content-Type: application/json

example/routes/person/_middleware.dart

@@ -27,6 +27,11 @@ class PersonValidator extends RequestValidator {
 
   @override
   List<ValidationRule> validationRules() => [
+        ValidationRule.headers(
+          HttpHeaders.contentTypeHeader,
+          (value) => value == 'application/json',
+          message: 'The API only works with content type `application/json`.',
+        ),
         ValidationRule.body(
           'username',
           (value) => value is String && value.isNotEmpty,