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/request.py

@@ -1,3 +1,4 @@
+import warnings
 from six import BytesIO, text_type
 from six.moves.urllib.parse import urlparse, parse_qsl
 from .util import CaseInsensitiveDict
@@ -6,23 +7,6 @@ from .util import CaseInsensitiveDict
 class Request(object):
     """
     VCR's representation of a request.
-
-    There is a weird quirk in HTTP.  You can send the same header twice.  For
-    this reason, headers are represented by a dict, with lists as the values.
-    However, it appears that HTTPlib is completely incapable of sending the
-    same header twice.  This puts me in a weird position: I want to be able to
-    accurately represent HTTP headers in cassettes, but I don't want the extra
-    step of always having to do [0] in the general case, i.e.
-    request.headers['key'][0]
-
-    In addition, some servers sometimes send the same header more than once,
-    and httplib *can* deal with this situation.
-
-    Futhermore, I wanted to keep the request and response cassette format as
-    similar as possible.
-
-    For this reason, in cassettes I keep a dict with lists as keys, but once
-    deserialized into VCR, I keep them as plain, naked dicts.
     """
 
     def __init__(self, method, uri, body, headers):
@@ -33,9 +17,7 @@ class Request(object):
             self.body = body.read()
         else:
             self.body = body
-        self.headers = CaseInsensitiveDict()
-        for key, value in headers.items():
-            self.add_header(key, value)
+        self.headers = headers
 
     @property
     def headers(self):
@@ -43,8 +25,8 @@ class Request(object):
 
     @headers.setter
     def headers(self, value):
-        if not isinstance(value, CaseInsensitiveDict):
-            value = CaseInsensitiveDict(value)
+        if not isinstance(value, HeadersDict):
+            value = HeadersDict(value)
         self._headers = value
 
     @property
@@ -58,11 +40,10 @@ class Request(object):
         self._body = value
 
     def add_header(self, key, value):
-        # see class docstring for an explanation
-        if isinstance(value, (tuple, list)):
-            self.headers[key] = value[0]
-        else:
-            self.headers[key] = value
+        warnings.warn("Request.add_header is deprecated. "
+                      "Please assign to request.headers instead.",
+                      DeprecationWarning)
+        self.headers[key] = value
 
     @property
     def scheme(self):
@@ -116,3 +97,35 @@ class Request(object):
     @classmethod
     def _from_dict(cls, dct):
         return Request(**dct)
+
+
+class HeadersDict(CaseInsensitiveDict):
+    """
+    There is a weird quirk in HTTP.  You can send the same header twice.  For
+    this reason, headers are represented by a dict, with lists as the values.
+    However, it appears that HTTPlib is completely incapable of sending the
+    same header twice.  This puts me in a weird position: I want to be able to
+    accurately represent HTTP headers in cassettes, but I don't want the extra
+    step of always having to do [0] in the general case, i.e.
+    request.headers['key'][0]
+
+    In addition, some servers sometimes send the same header more than once,
+    and httplib *can* deal with this situation.
+
+    Futhermore, I wanted to keep the request and response cassette format as
+    similar as possible.
+
+    For this reason, in cassettes I keep a dict with lists as keys, but once
+    deserialized into VCR, I keep them as plain, naked dicts.
+    """
+
+    def __setitem__(self, key, value):
+        if isinstance(value, (tuple, list)):
+            value = value[0]
+
+        # Preserve the case from the first time this key was set.
+        old = self._store.get(key.lower())
+        if old:
+            key = old[0]
+
+        super(HeadersDict, self).__setitem__(key, value)