T::S::Declare : name() will set $row_class unless it has already been
set.

Previously, name() would set $row_class no matter what. If row_class()
had been called before name(), the setting would be overwritten.
With this change, name() will set the row_class only if it hasn't been
set already.
Merge pull request #124 from GeJ/PR_do_not_override_rowclass

Teng::Schema::Declare#name : set $row_class only if it doesn't exist.