Writing NaN differs from nodejs Buffer

Hello,

I was recently attempting to swap out nodejs' Buffer with this one on a fork of another project, so I could use it in a non-node environment, when I noticed an inconsistency between nodejs' Buffer and this library when dealing with `NaN`

Here's a sample (using most recent npm install of 'buffer'):
```
const AltBuffer = require('buffer/').Buffer;

const alt_buf = new AltBuffer(8);
const node_buf = new Buffer(8);

alt_buf.writeDoubleLE(NaN, 0);
node_buf.writeDoubleLE(NaN, 0);

```
in node 6.x, 8.x, and 9.x, the resulting buffer objects are different

Node's (native) Buffer:
`00 00 00 00 00 00 f8 7f`

buffer library's Buffer:
`01 00 00 00 00 00 f0 7f` - bytes 0 and 6 are different

That's probably not supposed to happen, right?