add support for immutable cache-control response header

CHANGES.txt

diff --git a/CHANGES.txt b/CHANGES.txt
index 8b5a51c..b44298e 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,3 +1,10 @@
+unreleased
+----------
+
+- Support setting the ``Cache-Control: immutable`` response header via
+  ``response.cache_control.immutable = True`` and
+  ``response.cache_expires(immutable=True)``.
+
 1.8.0rc1 (2018-01-01)
 ---------------------
 

src/webob/cachecontrol.py

diff --git a/src/webob/cachecontrol.py b/src/webob/cachecontrol.py
index 9176ec9..45df7ba 100644
--- a/src/webob/cachecontrol.py
+++ b/src/webob/cachecontrol.py
@@ -205,6 +205,7 @@
     stale_while_revalidate = value_property(
         'stale-while-revalidate', type='response')
     stale_if_error = value_property('stale-if-error', type='response')
+    immutable = exists_property('immutable', type='response')
 
     def __str__(self):
         return serialize_cache_control(self.properties)

tests/test_cachecontrol.py

diff --git a/tests/test_cachecontrol.py b/tests/test_cachecontrol.py
index 04ddaa6..ce299d9 100644
--- a/tests/test_cachecontrol.py
+++ b/tests/test_cachecontrol.py
@@ -227,10 +227,11 @@
 
     def test_parse(self):
         from webob.cachecontrol import CacheControl
-        cc = CacheControl.parse("public, max-age=315360000")
+        cc = CacheControl.parse("public, max-age=315360000, immutable")
         assert type(cc) == CacheControl
         assert cc.max_age == 315360000
         assert cc.public is True
+        assert cc.immutable is True
 
     def test_parse_updates_to(self):
         from webob.cachecontrol import CacheControl