* fix(transport/http): responseEncoder should not write any data when it need to write nil
* test(transport/http): adjust the test of response encoder encode nil value
* fix(transport/http): remove content-type setting
The io/ioutil package has been deprecated as of Go 1.16, see
https://golang.org/doc/go1.16#ioutil. This commit replaces the existing
io/ioutil functions with their new definitions in io and os packages.
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>