fix(pl.path): make expanduser more sturdy

see Kong/lua-resty-aws#110

lua/pl/path.lua

@@ -493,12 +493,29 @@ end
 -- In windows, if HOME isn't set, then USERPROFILE is used in preference to
 -- HOMEDRIVE HOMEPATH. This is guaranteed to be writeable on all versions of Windows.
 -- @string P A file path
+-- @treturn[1] string The file path with the `~` prefix substituted, or the input path if it had no prefix.
+-- @treturn[2] nil
+-- @treturn[2] string Error message if the environment variables were unavailable.
 function path.expanduser(P)
     assert_string(1,P)
     if at(P,1) == '~' then
         local home = getenv('HOME')
-        if not home then -- has to be Windows
-            home = getenv 'USERPROFILE' or (getenv 'HOMEDRIVE' .. getenv 'HOMEPATH')
+        if (not home) and path.is_windows then
+            home = getenv 'USERPROFILE'
+            if not home then
+                local hd = getenv 'HOMEDRIVE'
+                local hp = getenv 'HOMEPATH'
+                if hd and hp then
+                    home = hd..hp
+                end
+            end
+            if not home then
+                return nil, "failed to expand '~' (HOME, USERPROFILE and HOMEDRIVE or HOMEPATH not set)"
+            end
+        else
+            if not home then
+                return nil, "failed to expand '~' (HOME not set)"
+            end
         end
         return home..sub(P,2)
     else