Swift 3 Query String from Dictionary

import Foudnation

extension Dictionary where Key: AnyHashable, Value: AnyHashable {
    
    var queryString: String? {
        var output: String = ""
        var i = 0
        for (key, i) in self.keys.enumerate() {
            output += prefix + "\(key)=\(self[value])"
        }
        return output
    }
}