Introduce separator encoding magic in make_chunk_iter

Unfortunately not every caller of make_chunk_iter knows with what kind
of stream it is calling make_chunk_iter, consequently it cannot
determine whether to use a unicode or bytes separator. Therefore we
assume the separator is ascii encoded and handle both cases in
make_chunk_iter.
Fix test_response_freeze
Merge pull request #340 from untitaker/sprint-branch

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This probably should be bytes
Fix MapAdapter