response header content-type might be undefined

[ozomer]

https://github.com/bithavoc/express-winston/blob/master/index.js#L201

res._headers['content-type'].indexOf('json') >= 0 ? JSON.parse(chunk) : chunk

res._headers['content-type'] might be undefined. indexOf throws an error.
The code should be replaced with:

(res._headers['content-type'] || '').indexOf('json') >= 0 ? JSON.parse(chunk) : chunk

Or in order to catch content-types that contain json\jsonp but not lalalajsonlalala:

/(\W|^)jsonp?(\W|$)/i.test(res._headers['content-type'] || '') ? JSON.parse(chunk) : chunk

Also, consider changing res._headers[...] with res.get(...).
Furthermore, In my opinion there is no need to wrap the JSON.parse with try-catch because I don't see a reason to use a json content-type and not returning a json object in the body...

[ozomer]

Just noticed #41