Add const_at to unordered map

Cython/Includes/libcpp/unordered_map.pxd

@@ -32,7 +32,8 @@ cdef extern from "<unordered_map>" namespace "std" nogil:
         bint operator>(unordered_map&, unordered_map&)
         bint operator<=(unordered_map&, unordered_map&)
         bint operator>=(unordered_map&, unordered_map&)
-        U& at(T&)
+        U& at(const T&)
+        const U& const_at "at"(const T&)
         iterator begin()
         const_iterator const_begin "begin"()
         void clear()

tests/run/cpp_stl_cpp11.pyx

@@ -151,6 +151,8 @@ def test_unordered_map_functionality():
         unordered_map[int,int].iterator iterator = int_map.begin()
         pair[unordered_map[int,int].iterator, bint] pair_iter  = int_map.insert(pair_insert)
         unordered_map[int, int] int_map2
+        unordered_map[int, int*] intptr_map
+        const int* intptr
     assert int_map[1] == 2
     assert int_map.size() == 1
     assert int_map.erase(1) == 1 # returns number of elements erased
@@ -178,4 +180,7 @@ def test_unordered_map_functionality():
     int_map.bucket_count()
     int_map.max_bucket_count()
     int_map.bucket(3)
+
+    intptr_map[0] = NULL
+    intptr = intptr_map.const_at(0)
     return "pass"