An exception is raised if req.header contains a property whose value is undefined

[mpvosseller]

If req.header contains a property whose value is undefined then the koa2-winston logger raises an exception with a stack trace like this:

TypeError: Cannot read property 'toString' of undefined
    at $asString (eval at build (/path/node_modules/fast-json-stringify/index.js:149:20), <anonymous>:16:15)
    at $mainreqheader (eval at build (/path/node_modules/fast-json-stringify/index.js:149:20), <anonymous>:371:44)
    at $mainreq (eval at build (/path/node_modules/fast-json-stringify/index.js:149:20), <anonymous>:229:17)
    at $main (eval at build (/path/node_modules/fast-json-stringify/index.js:149:20), <anonymous>:190:17)
    at Printf.template (/path/node_modules/koa2-winston/stringify_schema.js:204:21)
    at Printf.transform (/path/node_modules/logform/printf.js:11:26)
    at Format.transform (/path/node_modules/logform/combine.js:20:24)
    at DerivedLogger._transform (/path/node_modules/winston/lib/winston/logger.js:305:29)
    at DerivedLogger.Transform._read (/path/node_modules/winston/node_modules/readable-stream/lib/_stream_transform.js:177:10)
    at DerivedLogger.Transform._write (/path/node_modules/winston/node_modules/readable-stream/lib/_stream_transform.js:164:83)
[ERROR] 14:09:39 TypeError: Cannot read property 'toString' of undefined

I ran into this because another middleware I'm using sets a header property and sometimes the value of that property is undefined. I did submit a PR to that middleware to omit the property when its value is undefined but koa2-winston should probably handle such cases without throwing an exception.