v2: `process is not defined` error when bundled with webpack

[th0r]

The problem:
In our project we use Swagger UI which uses stream-browserify which uses this module, but version 2.

We bundle out app using Webpack and use node: {process: false} config option which prevents webpack to polyfill global process object. We have to do this for a couple of reasons (workaround some bugs in other dependencies).

But in this case this library throws process is not defined error because it assumes that global process object is defined.

Solution:
Add typeof process !== 'undefined' checks in problematic places. Here is the PR: #413

Notes:
This PR fixes v2 and was branched from v2.3.6 tag so it would be great to ship v2.3.7 with this fix.