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:
```
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"]]
```
