[now dev] Set Now response headers when proxy passing in routes

[TooTallNate]

This matches the behavior in production.

[codecov-io]

Codecov Report

Merging #2500 into canary will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           canary   #2500      +/-   ##
=========================================
+ Coverage   11.37%   11.4%   +0.02%     
=========================================
  Files         256     256              
  Lines        9247    9252       +5     
  Branches     1031    1032       +1     
=========================================
+ Hits         1052    1055       +3     
- Misses       8083    8085       +2     
  Partials      112     112

Impacted Files	Coverage Δ
src/util/dev/server.ts	47.59% <ø> (+0.33%)	⬆️
src/util/dev/builder-cache.ts	67.24% <0%> (-1.51%)	⬇️

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