box: updated httpc error message

Previously Tarantool used to raise the confusing error message in
case of invalid usage of the httpc module. Fixed to follow the
current module API.

Closes #4136

src/lua/httpc.lua

@@ -294,7 +294,7 @@ curl_mt = {
         --
         request = function(self, method, url, body, opts)
             if not method or not url then
-                error('request(method, url [, options]])')
+                error('request(method, url [, body, [options]])')
             end
             local resp = self.curl:request(method, url, body, opts or {})
             if resp and resp.headers then

test/app-tap/http_client.test.lua

@@ -62,10 +62,14 @@ local function stop_server(test, server)
 end
 
 local function test_http_client(test, url, opts)
-    test:plan(10)
+    test:plan(11)
 
     test:isnil(rawget(_G, 'http'), "global namespace is not polluted");
     test:isnil(rawget(_G, 'http.client'), "global namespace is not polluted");
+    local ok, err = pcall(client.request, client)
+    local usage_err = "request(method, url [, body, [options]])"
+    test:is_deeply({ok, tostring(err):split(': ')[2]}, {false, usage_err},
+                   "test httpc usage")
     local r = client.get(url, opts)
     test:is(r.status, 200, 'simple 200')
     test:is(r.reason, 'Ok', '200 - Ok')