Handling set_env correctly

Hello, 

Next to https://github.com/coreruleset/secrules_parsing/issues/90

There is still a problem here : 

This one is failing :

```
SecRule ARGS "@rx ^.{3,}" \
    "id:1,\
    phase:2,\
    pass,\
    setenv:my_env=my_env_value"
```

This one is considered clean :

```
SecRule ARGS "@rx ^.{3,}" \
    "id:1,\
    phase:2,\
    pass,\
    setenv:'my_env=my_env_value'"
```
