Allow nil in mw.text.jsonEncode

If it somehow gets in there (e.g. via a crafty __pairs), let it through.

Change-Id: I9f79dbb1a09cd62b2a8f4b6beb84a3e2f1c85560

engines/LuaCommon/lualib/mw.text.lua

@@ -308,7 +308,7 @@ local function checkForJsonEncode( t, seen, lvl )
 		if t ~= t or t >= math.huge or t <= -math.huge then
 			error( "mw.text.jsonEncode: Cannot encode non-finite numbers", lvl )
 		end
-	elseif tp ~= 'boolean' and tp ~= 'string' then
+	elseif tp ~= 'boolean' and tp ~= 'string' and tp ~= 'nil' then
 		error( string.format( "mw.text.jsonEncode: Cannot encode type '%s'", tp ), lvl )
 	end
 end

tests/engines/LuaCommon/TextLibraryTests.lua

@@ -430,6 +430,33 @@ local tests = {
 	  args = { '"foo"' },
 	  expect = { 'foo' }
 	},
+	{ name = 'json encode, sneaky nil injection (object)',
+	  func = mw.text.jsonEncode,
+	  args = { setmetatable( {}, {
+		  __pairs = function ( t )
+			  return function ( t, k )
+				  if k ~= "foo" then
+					  return "foo", nil
+				  end
+			  end, t, nil
+		  end,
+	  } ) },
+	  expect = { '{"foo":null}' }
+	},
+	{ name = 'json encode, sneaky nil injection (array)',
+	  func = mw.text.jsonEncode,
+	  args = { setmetatable( { "one", "two", nil, "four" }, {
+		  __pairs = function ( t )
+			  return function ( t, k )
+				  k = k and k + 1 or 1
+				  if k <= 4 then
+					  return k, t[k]
+				  end
+			  end, t, nil
+		  end,
+	  } ) },
+	  expect = { '["one","two",null,"four"]' }
+	},
 
 	{ name = 'json encode, invalid values (inf)',
 	  func = mw.text.jsonEncode,