allow []string values in map as well as []interface{} values

Semantically this should probably be v1.7, but the pull request with []string encoding came from left field based on manually creating a Map value rather than from NewMapXml or NewMapJson functions. I expect to fold it into the []interface{} marshaling "case" when I get some time. (It has never come up in over 4 years of hundreds of folks using this package.)