add unit test, new ITest and then fix the issue: different schema, bucket == skip
factored out the underlying logic for unit testing; also moved
maybeAddTrailingSlash to S3AUtils (while retaining/forwarnding existing method
in S3AFS).
tested: london, sole failure is
testListingDelete[auth=true](org.apache.hadoop.fs.s3a.ITestS3GuardOutOfBandOperations)
filed HADOOP-16853
Change-Id: I4b8d0024469551eda0ec70b4968cba4abed405ed
eaaaba12b1