Various fixes and tweaks

src/OAuth2/ResourceServer.php

@@ -168,7 +168,7 @@ class ResourceServer
         }
 
         if (empty($access_token)) {
-            throw new Exception\MissingAccessTokenException('Access Token is Missing');
+            throw new Exception\InvalidAccessTokenException('Access token is missing');
         }
 
         return $access_token;

src/OAuth2/Util/Request.php

@@ -66,8 +66,10 @@ class Request implements RequestInterface
     protected function readHeaders()
     {
         if (function_exists('getallheaders')) {
+            // @codeCoverageIgnoreStart
             $headers = getallheaders();
         } else {
+            // @codeCoverageIgnoreEnd
             $headers = array();
             foreach ($this->server() as $name => $value) {
                 if (substr($name, 0, 5) == 'HTTP_') {