add tests for streaming endpoint with fixed Content-Length

This previously caused an issue referenced here: https://github.com/excon/excon/commit/8e6457e86e8cf4e933d884068effa22d5f303ddc\#commitcomment-6513987

This issue was fixed in: https://github.com/excon/excon/commit/3f9d8122a8aed3c354f71e99bcef845d0487539f

Sorry for the inconvenience!

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>