Add support for SameSite cookies

The SameSite cookie attribute has yet to appear in an official RFC, and
until recently was exclusive to Chrome. However, Firefox has recently
implemented it as well, so it seems prudent to support it.

src/cow_cookie.erl

@@ -19,7 +19,8 @@
 
 -type cookie_option() :: {max_age, non_neg_integer()}
 	| {domain, binary()} | {path, binary()}
-	| {secure, boolean()} | {http_only, boolean()}.
+	| {secure, boolean()} | {http_only, boolean()}
+	| {same_site, lax | strict}.
 -type cookie_opts() :: [cookie_option()].
 -export_type([cookie_opts/0]).
 
@@ -215,8 +216,13 @@ setcookie(Name, Value, Opts) ->
 		{_, false} -> <<>>;
 		{_, true} -> <<"; HttpOnly">>
 	end,
+	SameSiteBin = case lists:keyfind(same_site, 1, Opts) of
+		false -> <<>>;
+		{_, lax} -> <<"; SameSite=Lax">>;
+		{_, strict} -> <<"; SameSite=Strict">>
+	end,
 	[Name, <<"=">>, Value, <<"; Version=1">>,
-		MaxAgeBin, DomainBin, PathBin, SecureBin, HttpOnlyBin].
+		MaxAgeBin, DomainBin, PathBin, SecureBin, HttpOnlyBin, SameSiteBin].
 
 -ifdef(TEST).
 setcookie_test_() ->
@@ -236,6 +242,12 @@ setcookie_test_() ->
 			[{secure, false}, {http_only, false}],
 			<<"Customer=WILE_E_COYOTE; Version=1">>},
 		{<<"Customer">>, <<"WILE_E_COYOTE">>,
+			[{same_site, lax}],
+			<<"Customer=WILE_E_COYOTE; Version=1; SameSite=Lax">>},
+		{<<"Customer">>, <<"WILE_E_COYOTE">>,
+			[{same_site, strict}],
+			<<"Customer=WILE_E_COYOTE; Version=1; SameSite=Strict">>},
+		{<<"Customer">>, <<"WILE_E_COYOTE">>,
 			[{path, <<"/acme">>}, {badoption, <<"negatory">>}],
 			<<"Customer=WILE_E_COYOTE; Version=1; Path=/acme">>}
 	],