Fix mtrand seed

src/Faker/Generator.php

@@ -145,7 +145,11 @@ public function getProviders()
 
     public function seed($seed = null)
     {
-        mt_srand($seed);
+        if ($seed === null) {
+            mt_srand();
+        } else {
+            mt_srand($seed);
+        }
     }
 
     public function format($formatter, $arguments = array())

test/Faker/GeneratorTest.php

@@ -105,6 +105,21 @@ public function testMagicCallCallsFormatWithArguments()
         $this->assertEquals('bazfoo', $generator->fooFormatterWithArguments('foo'));
     }
 
+    public function testSeed()
+    {
+        $generator = new Generator;
+
+        $generator->seed(0);
+        $mtRandWithSeedZero = mt_rand();
+        $generator->seed(0);
+        $this->assertEquals($mtRandWithSeedZero, mt_rand(), 'seed(0) should be deterministic.');
+
+        $generator->seed();
+        $mtRandWithoutSeed = mt_rand();
+        $this->assertNotEquals($mtRandWithSeedZero, $mtRandWithoutSeed, 'seed() should be different than seed(0)');
+        $generator->seed();
+        $this->assertNotEquals($mtRandWithoutSeed, mt_rand(), 'seed() should not be deterministic.');
+    }
 }
 
 class FooProvider