Add test that the local key takes precedence over an environment variable.

properties_test.go

diff --git a/properties_test.go b/properties_test.go
index 69e1cfc..3a30629 100644
--- a/properties_test.go
+++ b/properties_test.go
@@ -86,6 +86,7 @@
 	{"key=value\nkey2=${key}", "key", "value", "key2", "value"},
 	{"key=value\nkey2=${key}\nkey3=${key2}", "key", "value", "key2", "value", "key3", "value"},
 	{"key=${USER}", "key", os.Getenv("USER")},
+	{"key=${USER}\nUSER=value", "key", "value", "USER", "value"},
 }
 
 // define error test cases in the form of