Add missing request ID to context in norpc mode

[MattWhelan]

When self-hosting a Go lambda, the Request ID was missing from the invocation context.

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[codecov-commenter]

Codecov Report

Merging #313 into master will increase coverage by 0.39%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
+ Coverage   74.06%   74.46%   +0.39%     
==========================================
  Files          18       18              
  Lines         833      834       +1     
==========================================
+ Hits          617      621       +4     
+ Misses        152      150       -2     
+ Partials       64       63       -1     

Impacted Files	Coverage Δ
lambda/invoke_loop.go	74.54% <100.00%> (+6.02%)	⬆️

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[bmoffatt]

Thanks for catching this! I'm adding a test case to this PR before merging