Add exposeHeaders and allowedRequestHeaders that accept CharSequence, close #6328

Motivation:

Netty 4.1 introduced AsciiString and defines HttpHeaderNames constants
as such.

It would be convenient to be able to pass them to `exposeHeaders` and
`allowedRequestHeaders` directly without having to call `toString`.

Modifications:

Add `exposeHeaders` and `allowedRequestHeaders` overloads that take a
`CharSequence`.

Result:

More convenient API

codec-http/src/main/java/io/netty/handler/codec/http/cors/CorsConfigBuilder.java

@@ -148,6 +148,38 @@ public final class CorsConfigBuilder {
         return this;
     }
 
+    /**
+     * Specifies the headers to be exposed to calling clients.
+     *
+     * During a simple CORS request, only certain response headers are made available by the
+     * browser, for example using:
+     * <pre>
+     * xhr.getResponseHeader(HttpHeaderNames.CONTENT_TYPE);
+     * </pre>
+     *
+     * The headers that are available by default are:
+     * <ul>
+     * <li>Cache-Control</li>
+     * <li>Content-Language</li>
+     * <li>Content-Type</li>
+     * <li>Expires</li>
+     * <li>Last-Modified</li>
+     * <li>Pragma</li>
+     * </ul>
+     *
+     * To expose other headers they need to be specified which is what this method enables by
+     * adding the headers to the CORS 'Access-Control-Expose-Headers' response header.
+     *
+     * @param headers the values to be added to the 'Access-Control-Expose-Headers' response header
+     * @return {@link CorsConfigBuilder} to support method chaining.
+     */
+    public CorsConfigBuilder exposeHeaders(final CharSequence... headers) {
+        for (CharSequence header: headers) {
+            exposeHeaders.add(header.toString());
+        }
+        return this;
+    }
+
     /**
      * By default cookies are not included in CORS requests, but this method will enable cookies to
      * be added to CORS requests. Calling this method will set the CORS 'Access-Control-Allow-Credentials'
@@ -215,6 +247,29 @@ public final class CorsConfigBuilder {
         return this;
     }
 
+    /**
+     * Specifies the if headers that should be returned in the CORS 'Access-Control-Allow-Headers'
+     * response header.
+     *
+     * If a client specifies headers on the request, for example by calling:
+     * <pre>
+     * xhr.setRequestHeader('My-Custom-Header', "SomeValue");
+     * </pre>
+     * the server will receive the above header name in the 'Access-Control-Request-Headers' of the
+     * preflight request. The server will then decide if it allows this header to be sent for the
+     * real request (remember that a preflight is not the real request but a request asking the server
+     * if it allow a request).
+     *
+     * @param headers the headers to be added to the preflight 'Access-Control-Allow-Headers' response header.
+     * @return {@link CorsConfigBuilder} to support method chaining.
+     */
+    public CorsConfigBuilder allowedRequestHeaders(final CharSequence... headers) {
+        for (CharSequence header: headers) {
+            requestHeaders.add(header.toString());
+        }
+        return this;
+    }
+
     /**
      * Returns HTTP response headers that should be added to a CORS preflight response.
      *