Fix unit tests.

composer.json

@@ -4,6 +4,9 @@
     "keywords": [
             "content-security-policy",
             "csp",
+            "http",
+            "headers",
+            "security",
             "xss"
         ],
     "license":      "MIT",
@@ -31,6 +34,7 @@
         "paragonie/constant_time_encoding": "^2.0"
     },
     "require-dev": {
+        "psr/http-message": "^1",
         "phpunit/phpunit": "4.*|5.*"
     },
     "suggest": {

src/CSPBuilder.php

@@ -219,25 +219,26 @@ public function enableOldBrowserSupport(): self
      */
     public static function fromFile(string $filename = ''): self
     {
-        if (!file_exists($filename)) {
+        if (!\file_exists($filename)) {
             throw new \Exception($filename.' does not exist');
         }
-        $json = \file_get_contents($filename);
-        $array = \json_decode($json, true);
-        return new CSPBuilder($array);
+        return self::fromData(
+            \file_get_contents($filename)
+        );
     }
 
     /**
      * Factory method - create a new CSPBuilder object from a JSON data
      *
      * @param string $data
      * @return CSPBuilder
+     * @throws \Exception
      */
     public static function fromData($data = ''): self
     {
         $array = \json_decode($data, true);
 
-        if(!is_array($array)) {
+        if(!\is_array($array)) {
             throw new \Exception('Is not array valid');
         }
 
@@ -348,7 +349,7 @@ public function nonce(string $directive = 'script-src', string $nonce = ''): str
     {
         $ruleKeys = \array_keys($this->policies);
         if (!\in_array($directive, $ruleKeys)) {
-            return null;
+            return '';
         }
 
         if (empty($nonce)) {