Fix for #549 Add O_CREATE flag to the non-force branch of writeConfig

Also add test for the method

viper.go

@@ -1310,7 +1310,7 @@ func (v *Viper) writeConfig(filename string, force bool) error {
 		flags = os.O_CREATE | os.O_TRUNC | os.O_WRONLY
 	} else {
 		if _, err := os.Stat(filename); os.IsNotExist(err) {
-			flags = os.O_WRONLY
+			flags = os.O_CREATE | os.O_WRONLY
 		} else {
 			return fmt.Errorf("File: %s exists. Use WriteConfig to overwrite.", filename)
 		}

viper_test.go

@@ -987,6 +987,26 @@ p_ppu = 0.55
 p_batters.batter.type = Regular
 `)
 
+func TestSafeWriteConfigProperties(t *testing.T) {
+	v := New()
+	fs := afero.NewMemMapFs()
+	v.SetFs(fs)
+	v.SetConfigName("c")
+	v.SetConfigType("properties")
+	err := v.ReadConfig(bytes.NewBuffer(propertiesExample))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := v.SafeWriteConfigAs("c.properties"); err != nil {
+		t.Fatal(err)
+	}
+	read, err := afero.ReadFile(fs, "c.properties")
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, propertiesWriteExpected, read)
+}
+
 func TestWriteConfigProperties(t *testing.T) {
 	v := New()
 	fs := afero.NewMemMapFs()