Fix replaceextension from adding a leading dot

When an empty string was passed to replaceextension
as the new extension, it added a leading dot,
this update fixes/prevents it.

src/base/path.lua

@@ -231,7 +231,7 @@
 			return p
 		end
 
-		if not newext:findlast(".", true) then
+		if #newext > 0 and not newext:findlast(".", true) then
 			newext = "."..newext
 		end
 

tests/base/test_path.lua

@@ -420,6 +420,10 @@
 	function suite.getabsolute_replaceExtensionWithoutExtension()
 			test.isequal("/nunit/framework/main.foo", path.replaceextension("/nunit/framework/main",".foo"))
 	end
+    
+    function suite.getabsolute_replaceExtensionWithEmptyString()
+			test.isequal("foo", path.replaceextension("foo.lua",""))
+	end