httpc: allow only strings for opts.headers values

There are two reasons to disable support of tables with __tostring
metamethods as opts.headers values:

* It seems this feature is not much needed: a type convertion can be
  easily done on a user's end with explicit tostring() call if needed.
* It never did work since introduction in 2.1.1-311-g85e1d78bc ('httpc:
  add checking of headers in httpc:request'): __tostring presence was
  verified, but a function in the field was not invoked.

Now http client accepts only a Lua string as a header key or a value.

Closes #3679 (again).

src/lua/httpc.c

@@ -184,18 +184,9 @@ luaT_httpc_request(lua_State *L)
 		while (lua_next(L, -2) != 0) {
 			int header_type = lua_type(L, -1);
 			if (header_type != LUA_TSTRING) {
-				const char *err_msg =
-					"opts.headers values should be strings "
-					"or tables with \"__tostring\"";
-				if (header_type != LUA_TTABLE) {
-					httpc_request_delete(req);
-					return luaL_error(L, err_msg);
-				} else if (!luaL_getmetafield(L, -1,
-							      "__tostring")) {
-					httpc_request_delete(req);
-					return luaL_error(L, err_msg);
-				}
-				lua_pop(L, 1);
+				httpc_request_delete(req);
+				return luaL_error(L, "opts.headers values "
+						  "should be strings");
 			}
 			if (lua_type(L, -2) != LUA_TSTRING) {
 				httpc_request_delete(req);

test/app-tap/http_client.test.lua

@@ -273,15 +273,13 @@ local function test_errors(test)
     local r = http:get("http://do_not_exist_8ffad33e0cb01e6a01a03d00089e71e5b2b7e9930dfcba.ru")
 end
 
--- gh-3679 Check that opts.headers values can be strings or table
--- convertible to string only.
+-- gh-3679 Check that opts.headers values can be strings only.
 -- gh-4281 Check that opts.headers can be a table and opts.headers
 -- keys can be strings only.
 local function test_request_headers(test, url, opts)
     local exp_err_bad_opts_headers = 'opts.headers should be a table'
     local exp_err_bad_key = 'opts.headers keys should be strings'
-    local exp_err_bad_value = 'opts.headers values should be strings or ' ..
-                              'tables with "__tostring"'
+    local exp_err_bad_value = 'opts.headers values should be strings'
     local cases = {
         -- Verify opts.headers type checks.
         {
@@ -326,18 +324,6 @@ local function test_request_headers(test, url, opts)
             opts = {headers = {aaa = 'aaa'}},
             exp_err = nil,
         },
-        {
-            'header value with __tostring() metamethod',
-            opts = {headers = {aaa = setmetatable({}, {
-                __tostring = function(self)
-                    return 'aaa'
-                end})}},
-            exp_err = nil,
-            postrequest_check = function(opts)
-                assert(type(opts.headers.aaa) == 'table',
-                    '"aaa" header was modified in http_client')
-            end,
-        },
         {
             'boolean header value',
             opts = {headers = {aaa = true}},
@@ -359,15 +345,10 @@ local function test_request_headers(test, url, opts)
             exp_err = exp_err_bad_value,
         },
         {
-            'table header value w/o metatable',
+            'table header value',
             opts = {headers = {aaa = {}}},
             exp_err = exp_err_bad_value,
         },
-        {
-            'table header value w/o __tostring() metamethod',
-            opts = {headers = {aaa = setmetatable({}, {})}},
-            exp_err = exp_err_bad_value,
-        },
     }
     test:plan(#cases)