handle HTTP_1_1_REQUIRED stream GOAWAY error code by retrying on new HTTP/1.1 connection

HoneyryderChuckrequested to merge
gh-80 into master

it was previously only handling 421 status codes for the same effect; this achieves parity with the frame-driven redirection

Fixes https://github.com/HoneyryderChuck/httpx/issues/80

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