Added DefaultValue for query_ and post_param functions inside request_wrapper

Hi, I've added DefaultValue into query_param and post_param functions, so now we can use:
```erlang
Req:query_param("price", 10.0).
Req:post_param("title", "test").
```

I've also added simple param function, to be used when developer doesn't cares where data is coming from (GET/POST).