Fix query_string_normalizer

This fixes #11.

I was trying to set some `default_params` using the normal method:

```ruby
class Client
  include HTTMultiparty
  default_params({ token: 'fake'})
end
```

 and I saw them landing in my controller like:

```ruby
  { token: nil, fake: nil }
```

Totes weird.

Long story short, it was because you were smooshing everything into arrays when that's not what HTTParty needs/wants. The only thing that should be in an array are the file parameters. All non-file parameters should just be string versions of `"#{k}=#{v}"`. 

Super simple and easy fix.