Fix error due to missing header in the response in resolveSampling method

[jj22ee]

Issue #, if available:

The following error can occur when an incoming request header x-amzn-trace-id has Sampled=?. This can cause an error to occur for this logic specific for Sampled=?.

TypeError: Cannot set properties of undefined (setting 'x-amzn-trace-id')
    at Object.resolveSampling (/ <...> /aws-xray-sdk-node/packages/core/dist/lib/middleware/mw_utils.js:93:37)
    at Object.traceRequestResponseCycle (/ <...> /aws-xray-sdk-node/packages/core/dist/lib/middleware/mw_utils.js:153:14)

The root cause is that the traceRequestResponseCycle method is creating a copy of the response with Object.assign({}, res, { req: req }), but in doing so, res.header is not copied via Object.assign since it is a prototype method. Thus when this copied response is passed into resolveSampling, the error occurs when res.header is accessed.

The bug is introduced in this old commit - See Before and After.

• This old commit makes a copied value of the response to modify it, but doesn't do anything with it later. So it is not a useful change and is likely unintended. Since there doesn't seem to be any good reason for this specific change, this part could be reverted.

This traceRequestResponseCycle logic is used by aws-xray-sdk-express, aws-xray-sdk-restify, and aws-xray-sdk-fastify.

Also, res.header is a method, not an object which the SDK currently assumes.

• See header() usage in Express
• See header() usage in Restify

Description of changes:

• The response that is passed into the resolveSampling method in mw_utils.js is now the original response, which was the previous/original logic for Express and Restify. This is also the logic currently used in aws-xray-sdk-hapi and aws-xray-sdk-koa2.
• Add truthy check for res.header before it is modified in resolveSampling. This is needed because res.header is not guaranteed to exist, as is the case when this logic is used in aws-xray-sdk-fastify for Fastify users.
• Correctly use res.header as a method instead of treating it like an object.

Testing:

• Tested with local Express, Restify, and Fastify sample apps
  • Tested scenarios include incoming trace headers with and without Sampled=? cases. Ensure error occurs before the the fix, and there is no error after the fix. Ensure that the response headers include the XRay Context header when request headers have Sampled=? (not working for Fastify which doesn't provide header method in reply.raw).
• Add unit test for resolveSampling and traceRequestResponseCycle where Sampled=?.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.28%. Comparing base (72d523d) to head (eb8ae16).

Files with missing lines	Patch %	Lines
packages/core/lib/middleware/mw_utils.js	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #706      +/-   ##
==========================================
+ Coverage   83.56%   84.28%   +0.72%     
==========================================
  Files          36       36              
  Lines        1813     1814       +1     
==========================================
+ Hits         1515     1529      +14     
+ Misses        298      285      -13     

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