Add a setlist method to the Headers datastructure

The update method added in baa7bdc is
meant to replace headers with the mapping passed in. If a MultDict or
Headers object is passed in it would replace headers with the final
iterated value, rather than all the values iterated over. This could
lead to unexpected results, therefore this corrects the functionality
to what I think is expected.

Consider,

    h1 = Headers()
    h1.add("X-Multi", "value")
    h2 = Headers()
    h2.add("X-Multi", "newValue")
    h2.add("X-Multi", "alternativeValue")
    h1.update(h2)

previously `h1.getlist("X-Multi")` would likely equal
`["alternativeValue"]` whereas now it equals `["newValue",
"alternativeValue"]` which is as you'd expect.

src/werkzeug/datastructures.py

@@ -1235,6 +1235,19 @@ def set(self, _key, _value, **kw):
             return
         self._list[idx + 1 :] = [t for t in listiter if t[0].lower() != ikey]
 
+    def setlist(self, key, values):
+        """Set multiple header values at once.
+
+        The `values` argument should be iterable. This will replace
+        any existing values for the key with the values passed. It is
+        the inverse of the getlist method.
+
+        .. versionadded:: 1.0
+        """
+        self.set(key, values[0])
+        for value in values[1:]:
+            self.add(key, value)
+
     def setdefault(self, key, default):
         """Returns the value for the key if it is in the dict, otherwise it
         returns `default` and sets that value for `key`.
@@ -1279,10 +1292,21 @@ def update(self, *args, **kwargs):
             raise TypeError("update expected at most 1 arguments, got %d" % len(args))
 
         if args:
-            for key, value in iter_multi_items(args[0]):
-                self[key] = value
-
-        for key, value in iter_multi_items(kwargs):
+            mapping = args[0]
+
+            if isinstance(mapping, (Headers, MultiDict)):
+                for key in iterkeys(mapping):
+                    self.setlist(key, mapping.getlist(key))
+            elif isinstance(mapping, dict):
+                for key, value in iteritems(mapping):
+                    if isinstance(value, (tuple, list)):
+                        self.setlist(key, value)
+                    else:
+                        self[key] = value
+            else:
+                for item in mapping:
+                    self[key] = item
+        for key, value in iteritems(kwargs):
             self[key] = value
 
     def to_wsgi_list(self):

tests/test_datastructures.py

@@ -788,8 +788,11 @@ def test_update(self):
         assert h.getlist("y") == ["1"]
         h.update(z="2")
         assert h.getlist("z") == ["2"]
-        h.update(self.storage_class([("a", "b")]))
-        assert h["a"] == "b"
+        h2 = self.storage_class([("a", "b")])
+        h2.add("a", "c")
+        h.update(h2, d="e")
+        assert h.getlist("a") == ["b", "c"]
+        assert h["d"] == "e"
 
     def test_to_wsgi_list(self):
         h = self.storage_class()