Fix issue reading file.

Hi, thanks for porting this library, it's very useful!

I found a corner case that you might have not considered. It's not probably common, but it makes this library to not work in my case.

This is what I did to find this issue:

1. Inside my app, I use `godotenv.Read` to load the file into my own custom environment.
2. I use foreman to boot the application.

Since foreman already loads the `.env` file, the `Read` function returns an empty map.

I think it's more correct to return always the values in the file in the function `Read`, but put the ward to not override the environment for the function `Load.`

This change solves my issue, what do you think?