Remove generic parameter from BodyStream (#234)

I think `BodyStream` is more useful without being generic over the
request body.

I'm also looking into adding a response body from a stream called
`StreamBody` which will work pretty much opposite to this.

src/tests/mod.rs

@@ -698,3 +698,6 @@ where
 
     addr
 }
+
+pub(crate) fn assert_send<T: Send>() {}
+pub(crate) fn assert_sync<T: Sync>() {}