Adding a flag to disable automatic inclusion of x-requested-with

[rorticus]

Type: feature

The following has been addressed in the PR:

• [x] There is a related issue
• [x] All code matches the style guide
• [x] Unit or Functional tests are included in the PR

Description:

Adding an XHR request option, includeRequestedWith that defaults to true, that can be used to disable the automatic inclusion of the X-Requested-With.

Example,

xhr('some-url', { includeRequestedWith: false }).then(...)

Resolves #328