pass response data to completionHandler even upon error

[brunomacf]

Upon server error some information about error itself may come in data field of response. This change passes this data even upon error! In this case the client code can check for error in this way:

http.POST("login", parameters: params, completionHandler: {(response, error) in 
    if(error != nil) {
        println(response) // print response data that may describe the error.
    }
})

[cvasilak]

Hi @brunomacf thank you for these contributions!

Can I ask and will be easier for us if you can separate the last two commits in separate PR's cause it touches different parts and we can comment on it separately.

thanks again!

[corinnekrych]

Thanks @brunomacf for your contribution, I've created a separate PR #31 for your initial commit brunomacf@5e2b97b

I'm closing this PR in favor of the other one.
Regarding other commits could you explain a bit more the problem you want to solve and issue another PR.