Rerouted HTTP requests retain the original request headers

[brennerd11]

Description

When using the table_route decorator the headers of the original HTTP request issued to the initally targetted worker were lost.
Headers are now copied 1-to-1 before routing the request to the determined downstream worker

[codecov-commenter]

Codecov Report

Merging #276 (b382c48) into master (89c1614) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #276   +/-   ##
=======================================
  Coverage   94.29%   94.29%           
=======================================
  Files         100      100           
  Lines       10830    10830           
  Branches     1478     1478           
=======================================
  Hits        10212    10212           
  Misses        548      548           
  Partials       70       70           

Impacted Files	Coverage Δ
faust/app/router.py	96.92% <100.00%> (ø)

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