avoid unnessary array_merge calls

Author: digitalkaoz

src/JsonSchema/Constraints/Constraint.php

@@ -123,7 +123,9 @@ public function addError(JsonPointer $path = null, $message, $constraint='', arr
      */
     public function addErrors(array $errors)
     {
-        $this->errors = array_merge($this->errors, $errors);
+        if ($errors) {
+            $this->errors = array_merge($this->errors, $errors);
+        }
     }
 
     /**

src/JsonSchema/Constraints/Factory.php

@@ -128,7 +128,7 @@ public function setConstraintClass($name, $class)
      */
     public function createInstanceFor($constraintName)
     {
-        if (array_key_exists($constraintName, $this->constraintMap)) {
+        if (isset($this->constraintMap[$constraintName])) {
             if (!isset($this->instanceCache[$constraintName])) {
                 $this->instanceCache[$constraintName] = new $this->constraintMap[$constraintName](
                     $this->checkMode,

src/JsonSchema/Validator.php

@@ -9,7 +9,6 @@
 
 namespace JsonSchema;
 
-use JsonSchema\Constraints\SchemaConstraint;
 use JsonSchema\Constraints\Constraint;
 use JsonSchema\Entity\JsonPointer;