supports Object#to_json options

<code>[to_json](http://www.ruby-doc.org/stdlib-2.0/libdoc/json/rdoc/JSON.html)</code> is implemented by some core classes of Ruby and has its [option](http://www.ruby-doc.org/stdlib-2.0/libdoc/json/rdoc/JSON/Ext/Generator/State.html#new-method), which the gem does not currently support, this might be confusing to an end user who wants to use the gem in a project and still requires the default <code>to_json</code> behavior