fix incorrect parsing of complex rates

Refer to #18

Also, another cleaner way to solve this could be to remove line 117 in fix_response_json_data
```ruby
data.gsub!(Regexp.new("(\\\\x..|\\\\240)"), '')
```
Not sure if you need that for decode_basic_rate or to handle some legacy issues though.
Let me know if you'd prefer that other solution.