make hmac (mostly) compatible with stdlib hmac
diff --git a/tests/hazmat/primitives/test_hmac.py b/tests/hazmat/primitives/test_hmac.py
index e2b517a..81d9ac8 100644
--- a/tests/hazmat/primitives/test_hmac.py
+++ b/tests/hazmat/primitives/test_hmac.py
@@ -32,22 +32,26 @@
     )
 
     def test_hmac_reject_unicode(self, backend):
-        h = hmac.HMAC(key=b"mykey", hash_cls=hashes.SHA1, backend=backend)
+        h = hmac.HMAC(key=b"mykey", digestmod=hashes.SHA1, backend=backend)
         with pytest.raises(TypeError):
             h.update(six.u("\u00FC"))
 
     def test_base_hash_hexdigest_string_type(self, backend):
-        h = hmac.HMAC(key=b"mykey", hash_cls=hashes.SHA1, backend=backend,
-                      data=b"")
+        h = hmac.HMAC(key=b"mykey", digestmod=hashes.SHA1, backend=backend,
+                      msg=b"")
         assert isinstance(h.hexdigest(), str)
 
+    def test_hmac_no_digestmod(self):
+        with pytest.raises(ValueError):
+            hmac.HMAC(key=b"shortkey")
+
 
 class TestCopyHMAC(object):
     def test_copy_backend_object(self):
         pretend_hmac = pretend.stub(copy_ctx=lambda a: True)
         pretend_backend = pretend.stub(hmacs=pretend_hmac)
         pretend_ctx = pretend.stub()
-        h = hmac.HMAC(b"key", hashes.SHA1, backend=pretend_backend,
+        h = hmac.HMAC(b"key", digestmod=hashes.SHA1, backend=pretend_backend,
                       ctx=pretend_ctx)
         assert h._backend is pretend_backend
         assert h.copy()._backend is pretend_backend
diff --git a/tests/hazmat/primitives/utils.py b/tests/hazmat/primitives/utils.py
index 73a2469..c51fef5 100644
--- a/tests/hazmat/primitives/utils.py
+++ b/tests/hazmat/primitives/utils.py
@@ -93,11 +93,11 @@
     return test_base_hash
 
 
-def base_hash_test(backend, hash_cls, digest_size, block_size, only_if,
+def base_hash_test(backend, digestmod, digest_size, block_size, only_if,
                    skip_message):
     if only_if is not None and not only_if(backend):
         pytest.skip(skip_message)
-    m = hash_cls(backend=backend)
+    m = digestmod(backend=backend)
     assert m.digest_size == digest_size
     assert m.block_size == block_size
     m_copy = m.copy()
@@ -128,7 +128,7 @@
     assert m.hexdigest() == md.lower()
 
 
-def generate_hmac_test(param_loader, path, file_names, hash_cls,
+def generate_hmac_test(param_loader, path, file_names, digestmod,
                        only_if=None, skip_message=None):
     def test_hmac(self):
         for backend in _ALL_BACKENDS:
@@ -137,7 +137,7 @@
                     yield (
                         hmac_test,
                         backend,
-                        hash_cls,
+                        digestmod,
                         params,
                         only_if,
                         skip_message
@@ -145,17 +145,17 @@
     return test_hmac
 
 
-def hmac_test(backend, hash_cls, params, only_if, skip_message):
+def hmac_test(backend, digestmod, params, only_if, skip_message):
     if only_if is not None and not only_if(backend):
         pytest.skip(skip_message)
     msg = params[0]
     md = params[1]
     key = params[2]
-    h = hmac.HMAC(binascii.unhexlify(key), hash_cls)
+    h = hmac.HMAC(binascii.unhexlify(key), digestmod=digestmod)
     h.update(binascii.unhexlify(msg))
     assert h.hexdigest() == md
-    digest = hmac.HMAC(binascii.unhexlify(key), hash_cls,
-                       data=binascii.unhexlify(msg)).hexdigest()
+    digest = hmac.HMAC(binascii.unhexlify(key), digestmod=digestmod,
+                       msg=binascii.unhexlify(msg)).hexdigest()
     assert digest == md
 
 
@@ -172,11 +172,11 @@
     return test_base_hmac
 
 
-def base_hmac_test(backend, hash_cls, only_if, skip_message):
+def base_hmac_test(backend, digestmod, only_if, skip_message):
     if only_if is not None and not only_if(backend):
         pytest.skip(skip_message)
     key = b"ab"
-    h = hmac.HMAC(binascii.unhexlify(key), hash_cls)
+    h = hmac.HMAC(binascii.unhexlify(key), digestmod=digestmod)
     h_copy = h.copy()
     assert h != h_copy
     assert h._ctx != h_copy._ctx