Restore back-compat for timestamp input formats

While we want to encourage users to provide the appropriate
timezone info, this commit adds back the previous behavior, which
was to serialize the request assuming the user meant UTC.  I double
checked, and in prior versions, we weren't sending any tzinfo at all,
whereas now, we're at least explicitly specifying the UTC suffix if
none is provided.

Fixes aws/aws-cli#982

cc @kyleknap @danielgtaylor