Callback defined

[reegz]

Hi,

There appears to be an issue here where callback is defined, but is never brought in as an argument.

I got around this by introducing callback as an additional parameter in the constructor and then passing it through to the getBoundary function.

https://github.com/dudewheresmycode/node-lambda-multipart/blob/922d01c07a327f4e0e073d4ee8573220c418870c/index.js#L38