Bug fix - added auth_args method to MySQL::Diff::Database.

RT 79976: bin/mysqldiff was calling MySQL::Diff::Database->auth_args,
which did not exist. The stated workaround in the RT bug DB was to use
_auth_args_string directly. The solution was to just create a wrapper
method called auth_args. Added a unit test for regression checking.

Bug URL - https://rt.cpan.org/Public/Bug/Display.html?id=79976
Merge pull request #8 from estrabd/RT-79976-apply-missing-auth_args

Bug fix - added auth_args method to MySQL::Diff::Database.
Bug fix - fixed undefined value in duplicate field error message, M::D::Table.

RT 77002: lib/MySQL/Diff/Table.pm was improperly reporting the table
name when throwing an exception related to their being duplicate
field/column names during the parsing of the SQL table definition.

Fixed reference to object instance field and create a unit test to
check for regressions.