Excon streaming

[technoweenie]

This implements streaming response bodies in Excon. Confirmed with technoweenie/faraday-live#8, if you test this branch:

$ FARADAY_GEM_REF=excon_streaming docker-compose build tests
$ TEST_PROTO=https TEST_ADAPTER=excon TEST_METHOD=get docker-compose run tests

Note: Excon claims to yield the chunk, remaining bytes, and total bytes. In practice, it only seems to do this if the response came in as 1 chunk. Or maybe, it skips this on chunked responses that don't give the content size up front. Either way, Faraday has to maintain its own total byte count.

irb(main):003:0> Excon.get('http://geemus.com', response_block: lambda { |b,r,t| puts "RECV #{b.size}, rem #{r.to_i}/#{t.to_i} #{Digest::SHA1.hexdigest(b)}" });nil
RECV 3722, rem 0/3722 f5878b59cc37c7cc306a78cac34362ab52c72e46
=> nil
irb(main):004:0> Excon.get('https://jigsaw.w3.org/HTTP/ChunkedScript', response_block: lambda { |b,r,t| puts "RECV #{b.size}, rem #{r.to_i}/#{t.to_i} #{Digest::SHA1.hexdigest(b)}" });nil
RECV 9329, rem 0/0 61218bfa9a16320a916775562eece77bf4b82786
RECV 16304, rem 0/0 a547ac23bcd520b7c4fada5ac58aab3e781c7d07
RECV 2244, rem 0/0 25931bac8db4b27b866fd0d0ead8302a2d335c94
RECV 1428, rem 0/0 476afc2e63062d56f5e92fd5b167b06fd1c3e664
RECV 8560, rem 0/0 67dc7ba1c6123f67fde2dce96cd5e41cba317d6f
RECV 12836, rem 0/0 6bad2a8664449eae0bcf3bc6f0ae1b66c9584dc3
RECV 5712, rem 0/0 c011d596e925bf265f7660c12820499b3843b7dd
RECV 9980, rem 0/0 e92f6efb3d1e4e376580781cf04ea944bf5734d9
RECV 5807, rem 0/0 08e1d0def8b8a73542469fc95214049b9ffe6cec

[olleolleolle]

This reads great and makes sense - prepare the req_opts before using them.