Tar decompress works even if archive is not provided

We do not need provide archive on initialisation Tar object, as we can
provide link to the file to decompress directly in filter method.

Fixes #41

src/Compress/Tar.php

@@ -202,11 +202,12 @@ public function compress($content)
     public function decompress($content)
     {
         $archive = $this->getArchive();
-        if (empty($archive) || ! file_exists($archive)) {
+        if (file_exists($content)) {
+            $archive = str_replace(['/', '\\'], DIRECTORY_SEPARATOR, realpath($content));
+        } elseif (empty($archive) || ! file_exists($archive)) {
             throw new Exception\RuntimeException('Tar Archive not found');
         }
 
-        $archive = str_replace(['/', '\\'], DIRECTORY_SEPARATOR, realpath($content));
         $archive = new Archive_Tar($archive, $this->getMode());
         $target  = $this->getTarget();
         if (! is_dir($target)) {

test/Compress/TarTest.php

@@ -209,4 +209,32 @@ public function testTarToString()
         $filter = new TarCompression();
         $this->assertEquals('Tar', $filter->toString());
     }
+
+    /**
+     * @see https://github.com/zendframework/zend-filter/issues/41
+     */
+    public function testDecompressionDoesNotRequireArchive()
+    {
+        $filter = new TarCompression([
+            'archive' => $this->tmp . '/compressed.tar',
+            'target' => $this->tmp . '/zipextracted.txt',
+        ]);
+
+        $content = 'compress me ' . microtime(true);
+        $compressed = $filter->compress($content);
+
+        self::assertSame($this->tmp . DIRECTORY_SEPARATOR . 'compressed.tar', $compressed);
+
+        $target = $this->tmp;
+        $filter = new TarCompression([
+            'target' => $target,
+        ]);
+
+        $decompressed = $filter->decompress($compressed);
+        self::assertSame($target, $decompressed);
+        // per documentation, tar includes full path
+        $file = $target . DIRECTORY_SEPARATOR . $target . DIRECTORY_SEPARATOR . '/zipextracted.txt';
+        self::assertFileExists($file);
+        self::assertSame($content, file_get_contents($file));
+    }
 }