Fixed a failing spec when validation nil

A recent change which added URI.escape(value) meant that specs were failing when validating 'nil'.

We now only escape if it's a string, meaning all specs now pass again.

lib/validate_url.rb

@@ -20,7 +20,8 @@ def initialize(options)
       def validate_each(record, attribute, value)
         schemes = [*options.fetch(:schemes)].map(&:to_s)
         begin
-          uri = URI.parse(URI.escape(value))
+          escaped_uri = value ? URI.escape(value) : nil
+          uri = URI.parse(escaped_uri)
           host = uri && uri.host
           scheme = uri && uri.scheme