Removed parameters that isn't set from the send request url.

Hello. 

First of all thanks for application, it works great.

I found one problem with optional parameters of string type (parameters of other type works fine).
My api has string parameter lang, it is optional and null by default. 
If user doesn't enter this parameter in test client then it will be sent like lang={lang} that isn't very convinient. 

Probably would be better just ignore parameters without set values. 
I have added next code in SendRequest method (WebApiTestClient.js)
I am not very good in js, so probably it can be implemented in a better way.

Maybe this changes will be useful for other.
Thanks.