Address comments from reviews

* in #Pkg::Config::issue_reassignments
 - Addressed a bug where we assumed the return from an assignment was a
   sufficient judge of if a variable was set. If the variable was set
   intentionally to 'false' this will return unexpected results. Instead, check
   explicitly for 'nil.'

* in Pkg::Config::Params
 - Fixed up tabbing for hashes to be consistent

* in Pkg::Tar::tar
 - Made sure that we actually fail if we can't create a tarball, and warn the
   user about the flags we require of the 'tar' command.

* in Pkg::Util::Tool
 - is_git_repo? is more idiomatic than is_git_repo. To keep it backwards
   compatible, define the new one, but make an alias to the old one.

* in Pkg::Util::Serialization
 - In the spirite of future compatibility with other serialization formats, we
   should lazy load 'yaml' when we need, it not when we load the class

* Whitespace fixes

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>