Add HeadersDict, and mark add_header deprecated.

HeadersDict is a subclass of CaseInsensitiveDict with two new features:

  1. Preserve the case of the header key from the first time it was set.
     This means that later munging won't modify the key case. (You can
     force picking up the new case with `del` followed by setting.)

  2. If the value is a list or tuple, unpack it and store the first
     element. This is the same as how `Request.add_header()` used to work.

For backward compatibility this commit preserves `Request.add_header()` but
marks it deprecated.

vcr/stubs/__init__.py

@@ -188,8 +188,7 @@ class VCRConnection(object):
         log.debug('Got {0}'.format(self._vcr_request))
 
     def putheader(self, header, *values):
-        for value in values:
-            self._vcr_request.add_header(header, value)
+        self._vcr_request.headers[header] = values
 
     def send(self, data):
         '''