Allow to add non existent file if the content is provided

src/Box.php

@@ -226,10 +226,9 @@ function ($file): string {
      */
     public function addFile(string $file, string $contents = null, bool $binary = false): string
     {
-        Assertion::file($file);
-        Assertion::readable($file);
-
-        $contents = null === $contents ? file_get_contents($file) : $contents;
+        if (null === $contents) {
+            $contents = file_contents($file);
+        }
 
         $relativePath = make_path_relative($file, $this->basePath);
         $local = ($this->mapFile)($relativePath);

tests/BoxTest.php

@@ -107,6 +107,23 @@ public function test_it_can_add_a_file_to_the_phar(): void
         $this->assertSame($expectedContents, $actualContents);
     }
 
+    public function test_it_can_add_a_non_exitent_file_with_contents_to_the_phar(): void
+    {
+        $file = 'foo';
+        $contents = 'test';
+
+        $this->box->addFile($file, $contents);
+
+        $expectedContents = $contents;
+        $expectedPharPath = 'phar://test.phar/'.$file;
+
+        $this->assertFileExists($expectedPharPath);
+
+        $actualContents = file_get_contents($expectedPharPath);
+
+        $this->assertSame($expectedContents, $actualContents);
+    }
+
     public function test_it_can_add_a_file_with_absolute_path_to_the_phar(): void
     {
         $relativePath = 'path-to/foo';