add ability to pretty print response

it is very difficult to view the failure response, since everything is
printed inline.  this allows the options to ‘pretty print’ the JSON
output, just like is done in the ‘additional info’.

src/Illuminate/Foundation/Testing/Constraints/HasInDatabase.php

@@ -63,7 +63,7 @@ public function failureDescription($table)
     {
         return sprintf(
             "a row in the table [%s] matches the attributes %s.\n\n%s",
-            $table, $this->toString(), $this->getAdditionalInfo($table)
+            $table, $this->toString(JSON_PRETTY_PRINT), $this->getAdditionalInfo($table)
         );
     }
 
@@ -93,10 +93,11 @@ protected function getAdditionalInfo($table)
     /**
      * Get a string representation of the object.
      *
+     * @param  int  $options
      * @return string
      */
-    public function toString()
+    public function toString($options = 0)
     {
-        return json_encode($this->data);
+        return json_encode($this->data, $options);
     }
 }