Minor optimisations to lume.combine(), updated tests

lume.lua

@@ -400,8 +400,16 @@ end
 
 
 function lume.combine(...)
+  local n = select('#', ...)
+  if n == 0 then return noop end
+  if n == 1 then 
+    local fn = select(1, ...)
+    if not fn then return noop end
+    assert(iscallable(fn), "expected a function or nil")
+    return fn
+  end
   local funcs = {}
-  for i = 1, select("#", ...) do
+  for i = 1, n do
     local fn = select(i, ...)
     if fn ~= nil then
       assert(iscallable(fn), "expected a function or nil")

test/test_lume.lua

@@ -370,6 +370,12 @@ tests["lume.combine"] = function()
   fn = lume.combine(nil, a, nil, b, nil)
   fn(10, 20)
   testeq( acc, 230 )
+  local x = false
+  fn = lume.combine(function() x = true end)
+  fn()
+  testeq( x, true )
+  testeq( type(lume.combine(nil)), "function" )
+  testeq( type(lume.combine()),    "function" )
 end
 
 -- lume.call