Fixing maxBodyLength exceeded error form follow-redirects when axios's maxContentLength is not exceeded

Two if statements is unnecessary, we can replace with "if ... else ..."

This allows users of axios inside `electron` to provide the [`net`](https://electron.atom.io/docs/api/net/) module as the http transport instead of using nodes http/https modules.  This gives a whole bunch of things to Electron users including automatic proxy resolution.

* Adding a way to disable all proxy processing

When the proxy field in configuration is === false all proxy processing is
disabled. This specifically disable the 'http_proxy' environment variable
handling.

Fixes #635
Related to #434

* Change readme wording

From review comment on PR (#691)

* Update parseHeaders to match node http behavior

Node ignores duplicate entries for certain HTTP headers.

It also always converts the `set-cookie` header into an array.

* add tests for new duplicate header handling

* clarify comment

* Fixing baseURL not working in interceptors

* add test for  modify base URL in request interceptor

Since the http adapter is never used in the browser it's safe to use
the Buffer global and its isBuffer() method directly.

The is-buffer module checks if an object is a Buffer without causing
Webpack or Browserify to include the whole Buffer module in the bundle.

forEachMehtodNoData => forEachMethodNoData

* Fixing combineURLs to support an empty relativeURL

When combining the base and relative URLs, we should forego force
appending a slash to the base when the relative URL is empty.
This leads to a semantic url.

* Fixing combineURLs, allowing single slash relatives