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.

tests/unit/test_request.py

@@ -1,6 +1,6 @@
 import pytest
 
-from vcr.request import Request
+from vcr.request import Request, HeadersDict
 
 
 def test_str():
@@ -12,11 +12,16 @@ def test_headers():
     headers = {'X-Header1': ['h1'], 'X-Header2': 'h2'}
     req = Request('GET', 'http://go.com/', '', headers)
     assert req.headers == {'X-Header1': 'h1', 'X-Header2': 'h2'}
-
-    req.add_header('X-Header1', 'h11')
+    req.headers['X-Header1'] = 'h11'
     assert req.headers == {'X-Header1': 'h11', 'X-Header2': 'h2'}
 
 
+def test_add_header_deprecated():
+    req = Request('GET', 'http://go.com/', '', {})
+    pytest.deprecated_call(req.add_header, 'foo', 'bar')
+    assert req.headers == {'foo': 'bar'}
+
+
 @pytest.mark.parametrize("uri, expected_port", [
     ('http://go.com/', 80),
     ('http://go.com:80/', 80),
@@ -36,3 +41,30 @@ def test_uri():
 
     req = Request('GET', 'http://go.com:80/', '', {})
     assert req.uri == 'http://go.com:80/'
+
+
+def test_HeadersDict():
+
+    # Simple test of CaseInsensitiveDict
+    h = HeadersDict()
+    assert h == {}
+    h['Content-Type'] = 'application/json'
+    assert h == {'Content-Type': 'application/json'}
+    assert h['content-type'] == 'application/json'
+    assert h['CONTENT-TYPE'] == 'application/json'
+
+    # Test feature of HeadersDict: devolve list to first element
+    h = HeadersDict()
+    assert h == {}
+    h['x'] = ['foo', 'bar']
+    assert h == {'x': 'foo'}
+
+    # Test feature of HeadersDict: preserve original key case
+    h = HeadersDict()
+    assert h == {}
+    h['Content-Type'] = 'application/json'
+    assert h == {'Content-Type': 'application/json'}
+    h['content-type'] = 'text/plain'
+    assert h == {'Content-Type': 'text/plain'}
+    h['CONtent-tyPE'] = 'whoa'
+    assert h == {'Content-Type': 'whoa'}