(PEG-154) Improve format in DataContainer::toFormat
(PEG-154) Improve format in DataContainer::toFormat

Adding new line before appending a DataContainer nested into another
DataContainer.

MikaelSmith/cpp-pcp-client,MikaelSmith/cpp-pcp-client,puppetlabs/cpp-pcp-client,MikaelSmith/cpp-pcp-client,puppetlabs/cpp-pcp-client,MikaelSmith/cpp-pcp-client,puppetlabs/cpp-pcp-client,puppetlabs/cpp-pcp-client,MikaelSmith/cpp-pcp-client