Fix from base64.RawStdEncoding to base64.StdEncoding

[tkuchiki]

It seems that the BYTES type in Cloud Spanner needs to be encoded as base64 with padding.

https://github.com/googleapis/googleapis/blob/master/google/spanner/v1/type.proto#L116-L118
https://github.com/googleapis/google-cloud-go/blob/7eaaa470fda5dc7cd1ff041d6a898e35fb54920e/spanner/value.go#L1085

Currently, spanner-cli uses base64.RawStdEncoding, but the padding is missing when outputting the data.

Example of tests

Currently

$ spanner-cli -p <my-project> -i <my-instance> -d <my-db> -e 'SELECT id, bytes FROM TestTable'
id      bytes
1       aGVsbG8sIHdvcmxkIQ

Fixed

$ spanner-cli -p <my-project> -i <my-instance> -d <my-db> -e 'SELECT id, bytes FROM TestTable'
id      bytes
1       aGVsbG8sIHdvcmxkIQ==

[yfuruyama]

Thanks for fixing this!

Can you add a new test or fix this test so that we can check the padding?

[tkuchiki]

Thank you for your review!

OK, I will add a new fix to the test.

[tkuchiki]

I'm sorry for the late.
Fixed!

[yfuruyama]

Thanks for the fix! LGTM!