Common: fix flaky DecodeBase64Url test

[chuwy]

DecodeBase64UrlSpec sometimes fails with something like:

Error:    x decodeBase64Url should not return failure on any other string
Error:   Falsified after 94 passed tests.
Error:   > ARG_0: "ON"
Error:   > ARG_0_ORIGINAL: "榆࣑㛣㈢ݰ≙▔顐ᖐ쨺綞䇚海葚﷈ꥬ䆁速䳹ញ䵌굘កḦࢷ횽霎᥆㏲뇈↻O䬘䰦忨覽犆⼜먒ᚍመ츮堔㶣N덡絰튮뻺ᐻ嬎"
Error:   The seed is IdlEhpWy7WYChUK5JYqFXtlLoABSVxyy1r8xege5ZEE=
Error:   
Error:   > Left(could not base64 decode: Last encoded character (before the paddings if any) is a valid base 64 alphabet but not a possible value) is not Right (conversionUtilsSpecs.scala:219)

Seems to be introduced by #362

[chuwy]

Seems to be coming from Apache Commons: https://issues.apache.org/jira/browse/CODEC-263. Also the claim seems to be invalid - we don't expect any string to be a valid base64-encoded line.