Add headers option to defaultAJAXparams

[asnoeyink]

I was not able to use the XMLHttpRequest headers option until I added a headers: {} default to the defaultAJAXparams object. It seems that the option was getting lost when the other options were added to the ajaxParams variable in the for loop.

npm run build changed the dist files a bit, feel free to not take those changes.

[calvinmetcalf]

so I'm not sure with credentials option is going to be something we're going to want to have on by default

[asnoeyink]

Oh, yeah, definitely not. I was messing with it and didn't realize that it updated this pull request. :)

[ollyollyollyltd]

Is there any progress on merging this?

If the headers option is not present in defaultAJAXparams then it is discarded if added to the options, so currently adding headers to the options, as described in the documentation, does not have any effect.

[MarioSchuerewegen]

ah.  @ollyollyollyltd that is why it doesnt work...   been searching why..