InterfaceToVariable: Maintain backward compatibility

[radeksimko]

This was discovered when upgrading mitchellh/mapstructure to its new version which contains mitchellh/mapstructure#98 which in turn caused some tests in hashicorp/terraform to fail:

--- FAIL: TestInterpolateFuncMap (0.00s)
    --- FAIL: TestInterpolateFuncMap/${map("hello",_list("world"),_"what's",_list("up?"))} (0.00s)
        interpolate_funcs_test.go:2487: wrong result
            given: ${map("hello", list("world"), "what's", list("up?"))}
            got:   map[string]interface {}{"hello":map[string]interface {}{"Value":"world", "Type":"8"}, "what's":map[string]interface {}{"Value":"up?", "Type":"8"}}
            want:  map[string]interface {}{"hello":[]interface {}{"world"}, "what's":[]interface {}{"up?"}}
--- FAIL: TestInterpolateFuncMerge (0.00s)
    --- FAIL: TestInterpolateFuncMerge/${merge(map("a",_list("b")),_map("c",_list("d",_"e")))} (0.00s)
        interpolate_funcs_test.go:2487: wrong result
            given: ${merge(map("a", list("b")), map("c", list("d", "e")))}
            got:   map[string]interface {}{"a":map[string]interface {}{"Value":"b", "Type":"8"}, "c":map[string]interface {}{"Value":"e", "Type":"8"}}
            want:  map[string]interface {}{"a":[]interface {}{"b"}, "c":[]interface {}{"d", "e"}}
    --- FAIL: TestInterpolateFuncMerge/${merge(map("a",_var.maps[0]),_map("b",_list("c",_"d")))} (0.00s)
        interpolate_funcs_test.go:2487: wrong result
            given: ${merge(map("a", var.maps[0]), map("b", list("c", "d")))}
            got:   map[string]interface {}{"a":map[string]interface {}{"key1":"a", "key2":"b"}, "b":map[string]interface {}{"Value":"d", "Type":"8"}}
            want:  map[string]interface {}{"a":map[string]interface {}{"key2":"b", "key1":"a"}, "b":[]interface {}{"c", "d"}}
--- FAIL: TestInterpolateFuncJSONEncode (0.00s)
    --- FAIL: TestInterpolateFuncJSONEncode/${jsonencode(nestedlist)} (0.00s)
        interpolate_funcs_test.go:2487: wrong result
            given: ${jsonencode(nestedlist)}
            got:   "{\"Type\":\"64\",\"Value\":{\"Type\":\"8\",\"Value\":\"foo\"}}"
            want:  "[[\"foo\"]]"
FAIL
FAIL	github.com/hashicorp/terraform/config	0.296s

with this patch the mentioned tests pass:

$ go test -mod=vendor ./config
ok  	github.com/hashicorp/terraform/config	0.826s