Only work with the request target, not the line

As @evert notes, by allowing specifying the entire request line, we have
the possibility that it goes out of sync with the request method and
protocol version, which would lead to an inconsistent and invalid
message.

src/RequestInterface.php

@@ -21,56 +21,41 @@
 interface RequestInterface extends MessageInterface
 {
     /**
-     * Retrieves the message's request line.
+     * Retrieves the message's request target.
      *
-     * Retrieves the message's request line either as it will appear (for
+     * Retrieves the message's request-target either as it will appear (for
      * clients), as it appeared at request (for servers), or as it was
-     * specified for the instance (see withRequestLine()).
+     * specified for the instance (see withRequestTarget()).
      *
-     * This method MUST return a string of the form:
+     * In most cases, this will be the origin-form of the composed URI,
+     * unless a value was provided to the concrete implementation (see
+     * withRequestTarget() below).
      *
-     * <code>
-     * HTTP_METHOD REQUEST_TARGET HTTP/PROTOCOL_VERSION
-     * </code>
-     *
-     * If the request line is calculated at method execution (i.e., not from
-     * a value set on the instance), the request-target MUST be in origin-form.
-     *
-     * If any aspect of the request line is unknown, it MUST raise an
-     * exception.
+     * If no URI is available, and no request-target has been specifically
+     * provided, this method MUST return the string "/".
      *
      * @return string
-     * @throws \RuntimeException if unable to construct a valid request line.
      */
-    public function getRequestLine();
+    public function getRequestTarget();
 
     /**
-     * Create a new instance with a specific request line.
-     *
-     * If the request needs a specific request line — for instance, to allow
-     * specifying an absolute-form, authority-form, or asterisk-form
-     * request-target — this method may be used to create an instance with
-     * the specified request line, verbatim.
-     *
-     * This method MUST validate that the line is in the form:
-     *
-     * <code>
-     * HTTP_METHOD REQUEST_TARGET HTTP/PROTOCOL_VERSION
-     * </code>
+     * Create a new instance with a specific request-target.
      *
-     * and raise an exception if not.
+     * If the request needs a non-origin-form request-target — e.g., for
+     * specifying an absolute-form, authority-form, or asterisk-form —
+     * this method may be used to create an instance with the specified
+     * request-target, verbatim.
      *
      * This method MUST be implemented in such a way as to retain the
      * immutability of the message, and MUST return a new instance that has the
-     * changed request line.
+     * changed request target.
      *
      * @link http://tools.ietf.org/html/rfc7230#section-2.7 (for the various
      *     request-target forms allowed in request messages)
-     * @param mixed $requestLine
+     * @param mixed $requestTarget
      * @return self
-     * @throws \InvalidArgumentException for invalid request lines.
      */
-    public function withRequestLine($requestLine);
+    public function withRequestTarget($requestTarget);
 
     /**
      * Retrieves the HTTP method of the request.