Add encode_options to DMM::Client

`encode_params!` failed and cannot get response,
if keyword has character which is undefined by EUC-JP.

this commit provide a way of avoiding.

example:

```ruby
    @client = DMM.new(
      api_id: config["api_id"],
      affiliate_id: config["affiliate_id"],
      result_only: true,
      encode_options: {:invalid => :replace, :undef => :replace, :replace => " "}
    )
```
(convert irregular character to whitespace.)

refs. http://rurema.clear-code.com/2.1.0/method/String/i/encode.html