Split response body type into own function (#566)

This allows removing the hack where we create a Response with an empty `reader`,
then immediately mutate it to set the real reader. It also happens to allow us
to get rid of 3 fields of Response that were only used to pass information to
`stream_to_reader`.

I've tried to keep the structure and logic of the body calculation as close to
the same as possible, for ease of review and to avoid introducing bugs. I think
there are some followup fixes we can make to the logic, which will be made
easier by having it in a self contained function.

src/unit.rs

@@ -109,10 +109,6 @@ impl Unit {
         }
     }
 
-    pub fn is_head(&self) -> bool {
-        self.method.eq_ignore_ascii_case("head")
-    }
-
     pub fn resolver(&self) -> ArcResolver {
         self.agent.state.resolver.clone()
     }