NPE on requests with empty body #462

Hi there!
I've come across #313 PR where issue was in response with empty body.
I have the same issue, but with Request.
We have POST requests with empty body and when I try to execute it, i receive NPE on line 47 (entity.writeTo(os) in AllureHttpClientRequest.class
I set "null" to body while building request. After I changed it to empty string - everything works fine.
So, as I understand, NPE is thrown because body content type is not defined and it comes as "null".

Is it a bug or expected behaviour?