Add browser shim (noop) for isomorphic use.

[DylanPiercey]

This allows for the module to be consumed isomorphically. For example with "isomorphic-fetch" / "node-fetch".

The browser doesn't use http.Agent and this change just makes it so that bundlers like browserify/webpack/rollup will export this module as a noop.

[mention-bot]

By analyzing the blame information on this pull request, we identified @fengmk2 to be a potential reviewer

[fengmk2]

+1

[fengmk2]

2.2.0