Detail Host header sources

Per the [related PR for fig-standards](php-fig/fig-standards#446), this PR
updates the `RequestInterface` to override the `getHeader()` method and indicate
that the Host header should be pulled from the composed URI if no value has been
previously set.

Author: weierophinney

src/RequestInterface.php

@@ -20,6 +20,22 @@
  */
 interface RequestInterface extends MessageInterface
 {
+    /**
+     * Extends MessageInterface::getHeader() to provide request-specific
+     * behavior.
+     *
+     * This method acts exactly like MessageInterface::getHeader(), with
+     * one behavioral change: if the Host header is requested, but has
+     * not been previously set, the method SHOULD attempt to pull the host
+     * segment of the composed URI, if present.
+     *
+     * @see MessageInterface::getHeader()
+     * @see UriInterface::getHost()
+     * @param string $name Case-insensitive header field name.
+     * @return string
+     */
+    public function getHeader($name);
+
     /**
      * Retrieves the message's request target.
      *