Add getTrailer() method

Author: huangzhhui

src/Server/Response.php

@@ -38,7 +38,7 @@ public function withContent(string $content): self
  }
 
  /**
- * Return an instance with specified cookies.
+ * Returns an instance with specified cookies.
  */
  public function withCookie(Cookie $cookie): self
  {
@@ -48,20 +48,34 @@ public function withCookie(Cookie $cookie): self
  }
 
  /**
- * Return all cookies.
+ * Retrieves all cookies.
  */
  public function getCookies(): array
  {
  return $this->cookies;
  }
 
+ /**
+ * Returns an instance with specified trailer.
+ */
  public function withTrailer(string $key, $value): self
  {
  $new = clone $this;
  $new->trailers[$key] = $value;
  return $new;
  }
 
+ /**
+ * Retrieves a specified trailer value, returns null if the value does not exists.
+ */
+ public function getTrailer(string $key)
+ {
+ return $this->trailers[$key] ?? null;
+ }
+
+ /**
+ * Retrieves all trailers values.
+ */
  public function getTrailers(): array
  {
  return $this->trailers;