Use String#freeze to allocate a single string
Use String#freeze to allocate a single string

Upstream psych uses a constant to avoid creating so many strings.
We can use freeze in our codebase since we use ruby 2.2+

Fixes: Already initialized constant Psych::Visitors::ToRuby::SHOVEL
Fixes #5772

aljesusg/manageiq-gems-pending,rsevilla87/manageiq-gems-pending,aljesusg/manageiq-gems-pending,rsevilla87/manageiq-gems-pending