Fix the behavior of the encoder

When upgrading faraday_middleware I found the difference in the behavior of `Faraday::Utils.parse_query`

```ruby
# Faraday 0.8.5
Faraday::Utils.parse_query('params=1&access_token')
# => {"Params" => "1", "access_token" => nil}

# Faraday 0.9.0.pre
Faraday::Utils.parse_query('params=1&access_token')
# => {"Params" => "1", "access_token" => true}
```
Corrected this.