Change error into awesome user-friendlyness

Hi there,

I was getting a very weird error "ODict valuelist should be lists", when attempting to add a header.

My code was as followed (using libmproxy):

```
        msg.headers["API-Key"] = new_headers["API-Key"]                                                                                                                                                                              
 42         msg.headers["API-Sign"] = new_headers["API-Sign"]
```

In the end, that was because there could be multiple equal headers. In order to cater to that, it you guys might enjoy the patch I attach, for it converts strings automatically into lists of multiple headers.

I think it should work, but I haven't tested it :$

It'd allow me to have the above code, instead of this one below:

```
        msg.headers["API-Key"] = [new_headers["API-Key"]]                                                                                                                                                                               
 42         msg.headers["API-Sign"] = [new_headers["API-Sign"]]
```