Avoid "[...] MUST NOT have a body [...]" warnings
Avoid "[...] MUST NOT have a body [...]" warnings

* Don't write any body if it's empty
* This was a problem with eg. 304 responses, as they must not have a body

robohydra/robohydra,mlev/robohydra,mlev/robohydra,mlev/robohydra,robohydra/robohydra