Do not override set vars with falsy values

[marcoow]

Currently dotenv would override env vars that are set but are set to a falsy value, e.g. '':

cat .env
MYVAR=value

MYVAR='' node
> process.env.MYVAR
''
> const dotenv = require('dotenv');
> dotenv.config()
{ MYVAR: 'value' }

This changes that behavior so that any env var that is present in process.env will be kept, regardless of its value:

cat .env
MYVAR=value

MYVAR='' node
> process.env.MYVAR
''
> const dotenv = require('dotenv');
> dotenv.config()
{ MYVAR: '' }

I think this would have to be considered a breaking change and would require a major version update.

Closes #202

[coveralls]

Coverage remained the same at 100.0% when pulling b61c605 on simplabs:fix-override into 825c1b2 on motdotla:master.

[jcblw]

thanks for the PR