[Http] Add support for HTTP/1.1 continue status codes

Some clients need this before they send their request body, in order to
avoid sending huge amounts of data needlessly

Author: igorw

Request.php

@@ -50,6 +50,11 @@ public function getHeaders()
         return $this->headers;
     }
 
+    public function expectsContinue()
+    {
+        return isset($this->headers['Expect']) && '100-continue' === $this->headers['Expect'];
+    }
+
     public function isReadable()
     {
         return $this->readable;

Response.php

@@ -24,6 +24,15 @@ public function isWritable()
         return $this->writable;
     }
 
+    public function writeContinue()
+    {
+        if ($this->headWritten) {
+            throw new \Exception('Response head has already been written.');
+        }
+
+        $this->conn->write("HTTP/1.1 100 Continue\r\n");
+    }
+
     public function writeHead($status = 200, array $headers = array())
     {
         if ($this->headWritten) {