The returned GoString() for a cty.ObjectWithOptionalAttrs was a bit off;
the returned slice of optional attributes was twice as long as it needed
to be due to a mismatch where the slice was created with make, but then
appended to (as opposed to indexed into).

I didn't see test coverage for Type GoString(), so I copied the test cases from Value's GoString() (fewer test cases, since I didn't need to differentiate between null and empty values). Here's the related test before the fix:

    --- FAIL: TestTypeGoString/cty.ObjectWithOptionalAttrs(map[string]cty.Type{"bar":cty.String,_"foo":cty.Bool},_[]string{"",_"bar"}) (0.00s)
        type_test.go:147: wrong result
            got:  cty.ObjectWithOptionalAttrs(map[string]cty.Type{"bar":cty.String, "foo":cty.Bool}, []string{"", "bar"})
            want: cty.ObjectWithOptionalAttrs(map[string]cty.Type{"bar":cty.String, "foo":cty.Bool}, []string{"bar"})