Add cow_http_hd:parse_accept_encoding/1

From RFC7231.

src/cow_http_hd.erl

@@ -16,6 +16,7 @@
 
 -export([parse_accept/1]).
 -export([parse_accept_charset/1]).
+-export([parse_accept_encoding/1]).
 -export([parse_connection/1]).
 -export([parse_content_length/1]).
 -export([parse_expect/1]).
@@ -317,6 +318,41 @@ horse_parse_accept_charset() ->
 	).
 -endif.
 
+%% @doc Parse the Accept-Encoding header.
+
+-spec parse_accept_encoding(binary()) -> [{binary(), qvalue()}].
+parse_accept_encoding(Encoding) ->
+	conneg_list(Encoding, []).
+
+-ifdef(TEST).
+parse_accept_encoding_test_() ->
+	Tests = [
+		{<<>>, []},
+		{<<"*">>, [{<<"*">>, 1000}]},
+		{<<"compress, gzip">>, [
+			{<<"compress">>, 1000},
+			{<<"gzip">>, 1000}
+		]},
+		{<<"compress;q=0.5, gzip;q=1.0">>, [
+			{<<"compress">>, 500},
+			{<<"gzip">>, 1000}
+		]},
+		{<<"gzip;q=1.0, identity; q=0.5, *;q=0">>, [
+			{<<"gzip">>, 1000},
+			{<<"identity">>, 500},
+			{<<"*">>, 0}
+		]}
+	],
+	[{V, fun() -> R = parse_accept_encoding(V) end} || {V, R} <- Tests].
+-endif.
+
+-ifdef(PERF).
+horse_parse_accept_encoding() ->
+	horse:repeat(20000,
+		parse_accept_encoding(<<"gzip;q=1.0, identity; q=0.5, *;q=0">>)
+	).
+-endif.
+
 %% @doc Parse the Connection header.
 
 -spec parse_connection(binary()) -> [binary()].