Add support to ES6 modules for the default agent

[topicus]

Because the exported HttpAgent is not exported under the default namespace the following code makes HttpAgent to be undefined

import HttpAgent, {HttpsAgent, HttpOptions, HttpsOptions} from "agentkeepalive";
console.log(HttpAgent) // undefinded

[codecov[bot]]

Codecov Report

Merging #75 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #75   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         174    175    +1     
=====================================
+ Hits          174    175    +1

Impacted Files	Coverage Δ
index.js	100% <100%> (ø)	:arrow_up:

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[codecov[bot]]

Codecov Report

Merging #75 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #75   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         174    175    +1     
=====================================
+ Hits          174    175    +1

Impacted Files	Coverage Δ
index.js	100% <100%> (ø)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 48e01a4...14ab9d6. Read the comment docs.

[fengmk2]

nodejs is not support es6 module by default at the current moment, I don't know which is the way to let commonjs support es6 module.

[topicus]

Closing this because is not correct. I was using typescript and to enable module interoperability it should set this flag in the compiler tsc --esModuleInterop.