fix uppercase in dateTime

Author: Antoine GUENEAU

.gitignore

@@ -8,3 +8,4 @@ coverage
 build
 .php_cs.cache
 .phpunit.result.cache
+.idea/

ReadMe.md

@@ -56,7 +56,7 @@ $purchase = new PurchaseRequest([
     'email' => 'john@snow.stark',
     'amount' => 42,
     'currency' => 'EUR',
-    'datetime' => new DateTime(),
+    'dateTime' => new DateTime(),
     'successUrl' => 'http://localhost/thanks',
     'errorUrl' => 'http://localhost/oops',
 ]);
@@ -194,7 +194,7 @@ $monetico = new Monetico(
 );
 
 $refund = new RefundRequest([
-    'datetime' => new DateTime(),
+    'dateTime' => new DateTime(),
     'orderDatetime' => new DateTime(),
     'recoveryDatetime' => new DateTime(),
     'authorizationNumber' => '1222',