Validator::rule now accepts any kind of callback

This has been achieved by making sure that the rule is neither
a "native" Validator rule (e.g. there is a validate{$CamleCaseName}
method) nor it has been registered.

If both conditions are met, the rule (even if it is a string)
will be accepted and used.

Tests for this case have also been added.

src/Valitron/Validator.php

@@ -1066,6 +1066,20 @@ class Validator
         return $name;
     }
 
+    /**
+     * Returns true if either a valdiator with the given name has been
+     * registered or there is a default validator by that name.
+     *
+     * @param string    $name
+     * @return bool
+     */
+    public function hasValidator($name)
+    {
+        $rules = $this->getRules();
+        return method_exists($this, "validate" . ucfirst($name))
+            || isset($rules[$name]);
+    }
+
     /**
      * Convenience method to add a single validation rule
      *
@@ -1079,12 +1093,8 @@ class Validator
         // Get any other arguments passed to function
         $params = array_slice(func_get_args(), 2);
 
-        // Note: we cannot use is_callable here since max, int, and many
+        if (is_callable($rule)
-        // other string can also be callables but aren't really rule callbacks.
+            && !(is_string($rule) && $this->hasValidator($rule)))
-        //
-        // If a closure is used, we can be sure that the user actually
-        // wants $rule to be a custom rule check.
-        if (is_object($rule) && ($rule instanceof \Closure))
         {
             $name = $this->getUniqueRuleName($fields);
             $msg = isset($params[0]) ? $params[0] : null;

tests/Valitron/ValidateAddInstanceRuleTest.php

@@ -1,6 +1,11 @@
 <?php
 use Valitron\Validator;
 
+function callbackTestFunction($item, $value)
+{
+	return $value === "bar";
+}
+
 class ValidateAddInstanceRuleTest extends BaseTestCase
 {
     protected function assertValid($v)
@@ -87,6 +92,20 @@ class ValidateAddInstanceRuleTest extends BaseTestCase
         $this->assertEquals("Foo test error message", $errors["foo"][0]);
     }
 
+	public function testAddRuleWithNamedCallbackOk()
+	{
+		$v = new Validator(array("bar" => "foo"));
+		$v->rule("callbackTestFunction", "bar");
+		$this->assertFalse($v->validate());
+	}
+
+	public function testAddRuleWithNamedCallbackErr()
+	{
+		$v = new Validator(array("foo" => "bar"));
+		$v->rule("callbackTestFunction", "foo");
+		$this->assertTrue($v->validate());
+	}
+
     public function testUniqueRuleName()
     {
         $v = new Validator(array());