Infer domain name from request context

[RaeesBhatti]

Issue #, if available:
Fixes: #46
Fixes: #13
Description of changes:
aws-lambda-go recently added support for DomainName property in RequestContext. We want to revert to that domain name rather than aws-serverless-go-api.com. This change is backwards compatible, in that, people using GO_API_HOST env variable can still use that.

I wasn't able to update dependencies properly with govendor because it keeps thowing an error when I try to do govendor get github.com/aws/aws-lambda-go@v1.17.0:

go: finding module for package github.com/aws/aws-lambda-go
can't load package: package github.com/aws/aws-lambda-go: module github.com/aws/aws-lambda-go@latest found (v1.17.0), but does not contain package github.com/aws/aws-lambda-go
Error: exit status 1

Please update dependencies manually and publish them here when reviewing this PR!

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

[RaeesBhatti]

@sapessi Is this a dead project? Should the community look for other solutions? Because I don't see any kind of review of feedback!

[sapessi]

Apologies @RaeesBhatti, I hadn't looked at this because I saw that the checks failed. Let me take a quick look

[RaeesBhatti]

Thank you! The tests are failing because of dependency issues. I couldn't update to github.com/aws/aws-lambda-go@v1.17.0 because govendor kept failing on me for some reason. Please update that dependency and push to this branch and the tests should pass.

[RaeesBhatti]

@sapessi I updated github.com/aws/aws-lambda-go/events. Can you please take a look again!

[sapessi]

Looks like the build is still failing. I suspect this is because the go.mod file is not updated to point to the latest version of the dependency, I only see a change in the sum file.

[RaeesBhatti]

@sapessi I've updated the mod file and test. They're passing now. Can you take another look please!