Regression 0.9.1 -> 0.10.0: .env parsed incorrectly

Hello!

I have upgraded dotenv from 0.9.1 to 0.10 and now my config file is parsed incorrectly.

Example file:

```
$ cat .env
VAR_A=

VAR_B=123
```

Current behaviour on 0.10:
```
$ dotenv list

VAR_A=VAR_B=123
```

Previous (correct) behaviour on 0.9:

```
$ dotenv list

VAR_A=
VAR_B=123
```