Add windows implementation of Clearenv for tests

Apparently `Clearenv` in Windows just sets the variables to ""

context_test.go

@@ -191,7 +191,7 @@ func TestContext_IsSet_fromEnv(t *testing.T) {
 		unparsableIsSet, uIsSet bool
 	)
 
-	os.Clearenv()
+	clearenv()
 	os.Setenv("APP_TIMEOUT_SECONDS", "15.5")
 	os.Setenv("APP_PASSWORD", "")
 	a := App{
@@ -256,7 +256,7 @@ func TestContext_GlobalIsSet_fromEnv(t *testing.T) {
 		unparsableIsSet, uIsSet bool
 	)
 
-	os.Clearenv()
+	clearenv()
 	os.Setenv("APP_TIMEOUT_SECONDS", "15.5")
 	os.Setenv("APP_PASSWORD", "")
 	a := App{

helpers_unix_test.go

@@ -0,0 +1,9 @@
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package cli
+
+import "os"
+
+func clearenv() {
+	os.Clearenv()
+}

helpers_windows_test.go

@@ -0,0 +1,20 @@
+package cli
+
+import (
+	"os"
+	"syscall"
+)
+
+// os.Clearenv() doesn't actually unset variables on Windows
+// See: https://github.com/golang/go/issues/17902
+func clearenv() {
+	for _, s := range os.Environ() {
+		for j := 1; j < len(s); j++ {
+			if s[j] == '=' {
+				keyp, _ := syscall.UTF16PtrFromString(s[0:j])
+				syscall.SetEnvironmentVariable(keyp, nil)
+				break
+			}
+		}
+	}
+}