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:

```
        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"]]
```

1 files changed, 3 insertions, 1 deletions

diff --git a/netlib/odict.py b/netlib/odict.py
index 46b74e8e..d0ff5cf6 100644
--- a/netlib/odict.py
+++ b/netlib/odict.py
@@ -60,7 +60,9 @@ class ODict:
             key, they are cleared.
         """
         if isinstance(valuelist, basestring):
-            raise ValueError("ODict valuelist should be lists.")
+            # convert the string into a single element list.
+            valuelist = [valuelist]
+            
         new = self._filter_lst(k, self.lst)
         for i in valuelist:
             new.append([k, i])