Fix for Issue #1354 (OctoPackAppendToPackageId)

Implemented a fix to allow the OctoPackAppendToPackageId command-line parameter to be used for projects that include a NuSpec file.

The previous modification of the Project Id was refactored out to a specific method and moved to later in the process such that it could apply whether an existing NuSpec file was available or not.

The `ShouldAllowAppendingValueToPackageId` test was also updated to allow for the new functionality.