Enable null value test for HTTP Get

[rajsite]

A test that verified null values in the middle of binary HTTP Get response was allowed through was disabled in #665. The test relied on a seeded random string of binary bytes that contained a null value in the content.

The migration to Python 3 broke the seed in httpbin's /bytes endpoint as discussed here: postmanlabs/httpbin#598

In addition there are not other clear endpoints for httpbin that can return well-specified binary data. It seemed possible that the base64/ endpoint would allow that but this was not the case: postmanlabs/httpbin#599

We should find a way to reliably test round tripping null values in the HTTP Client library to avoid regressions.