Add support for multi value header responses

[stealthycoin]

API Gateway added support for a multi-value header dictionary whcih can
be returned. This commit adds support for it in Chalice.

implements #1205

API Gateway docs: https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html#api-gateway-simple-proxy-for-lambda-output-format

[codecov-io]

Codecov Report

Merging #1214 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1214      +/-   ##
==========================================
+ Coverage   96.04%   96.04%   +<.01%     
==========================================
  Files          28       28              
  Lines        5229     5238       +9     
  Branches      668      670       +2     
==========================================
+ Hits         5022     5031       +9     
  Misses        134      134              
  Partials       73       73

Impacted Files	Coverage Δ
chalice/app.py	97.46% <100%> (+0.02%)	⬆️

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[stealthycoin]

The other option here would be to overload the headers dict and just look at if its a string or an list. Would be a better API. The reason I didn't do that is last time we tried to be clever and preserve backwards compatibility we caused problems that needed fixing.

Thoughts? This case seems simpler than the last one (#1165)

[jamesls]

The reason I didn't do that is last time we tried to be clever and preserve backwards compatibility we caused problems that needed fixing.

Are you referring to #1165? If so, I think that's a little different in the sense that we're not changing any existing behavior and just supporting a case we previously weren't (a list value in the headers dict). Given we're not messing around with types or anything, I think this is much safer.

Plus, reading the doc page you linked, the interaction between headers and multiValueHeaders seems tricky and could be confusing fore people.

My preference is to just make a list-value automatically work. What do others think?

[kyleknap]

My preference is to just make a list-value automatically work. What do others think?

Yeah let's just make lists work because:

1. I agree with James it is a safer change than the request one.
2. It is consistent with what we do for requests and would be more intuitive.