HTTP Propagation: set `Correlation-Context` header

[mayurkale22]

Continuation of https://github.com/census-instrumentation/opencensus-node/pull/445 and Fixes #297

Specs: https://github.com/census-instrumentation/opencensus-specs/blob/master/tags/TagMap.md#over-http

[codecov-io]

Codecov Report

Merging #567 into master will increase coverage by <.01%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #567      +/-   ##
=========================================
+ Coverage    95.3%   95.3%   +<.01%     
=========================================
  Files         148     148              
  Lines       10590   10656      +66     
  Branches      746     749       +3     
=========================================
+ Hits        10093   10156      +63     
- Misses        497     500       +3

Impacted Files	Coverage Δ
src/stackdriver-monitoring.ts	77.02% <0%> (-2.71%)	:arrow_down:
src/http.ts	90.3% <0%> (-0.06%)	:arrow_down:
src/binary-format.ts	100% <0%> (ø)	:arrow_up:
test/test-http.ts	98.86% <0%> (+0.24%)	:arrow_up:

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