Fix issue with ArrayBuffer

[nickuraltsev]

I think we should not wrap an ArrayBuffer in a DataView. XMLHttpRequest#send(ArrayBuffer) is deprecated, but XMLHttpRequest#send(ArrayBufferView) does not seem to be supported by some browsers like IE 11. BTW, fetch sends an ArrayBuffer as is, so my suggestion is to do the same.

This PR fixes #268.

[coveralls]

Coverage decreased (-0.05%) to 91.317% when pulling 4d83ba9 on nickuraltsev:array-buffer-fix into 5c58b83 on mzabriskie:master.

[mzabriskie]

What's the potential fall out for using ArrayBuffer if it is deprecated? Seems like this will work short term, but at some point will cause problems.

Note: Be aware to stop using a plain ArrayBuffer as parameter. This is not part of the XMLHttpRequest specification any longer. Use an ArrayBufferView instead (see compatibility table for version information).

[nickuraltsev]

@mzabriskie Yes, I saw that. The problem is that the deprecated API seems to have a better browser support than the new API (see below). This PR actually allows users to choose what to use - ArrayBuffer or ArrayBufferView (instead of forcing them to use ArrayBufferView which does not seem to be supported by IE).

[mzabriskie]

So in the case of wanting to use ArrayBufferView they would have to pass that in explicitly, instead of us converting ArrayBuffer for them, as is changed in this PR?

[nickuraltsev]

Right. They will be able to choose between ArrayBuffer and ArrayBufferView as if they were using XMLHttpRequest directly.