Fix implementation for hash tags extraction from keys.

We now fully comply with the specifications defined by Redis.

lib/Predis/Cluster/RedisClusterHashStrategy.php

@@ -303,8 +303,8 @@ class RedisClusterHashStrategy implements CommandHashStrategyInterface
     protected function extractKeyTag($key)
     {
         if (false !== $start = strpos($key, '{')) {
-            if (false !== $end = strpos($key, '}', $start)) {
-                $key = substr($key, ++$start, $end - $start);
+            if (false !== ($end = strpos($key, '}', $start)) && $end !== ++$start) {
+                $key = substr($key, $start, $end - $start);
             }
         }
 

tests/Predis/Cluster/RedisClusterHashStrategyTest.php

@@ -29,7 +29,14 @@ class RedisClusterHashStrategyTest extends PredisTestCase
         $this->assertSame(44950, $strategy->getKeyHash('{foo}'));
         $this->assertSame(44950, $strategy->getKeyHash('{foo}:bar'));
         $this->assertSame(44950, $strategy->getKeyHash('{foo}:baz'));
-        $this->assertSame(44950, $strategy->getKeyHash('bar:{foo}:bar'));
+        $this->assertSame(44950, $strategy->getKeyHash('bar:{foo}:baz'));
+        $this->assertSame(44950, $strategy->getKeyHash('bar:{foo}:{baz}'));
+
+        $this->assertSame(44950, $strategy->getKeyHash('bar:{foo}:baz{}'));
+        $this->assertSame(9415,  $strategy->getKeyHash('{}bar:{foo}:baz'));
+
+        $this->assertSame(0,     $strategy->getKeyHash(''));
+        $this->assertSame(31641, $strategy->getKeyHash('{}'));
     }
 
     /**