Make AS::SafeBuffer#prepend act like String#prepend

Make `#prepend` method modify instance in-place and return self
instead of just returning modified value. That is exactly what
`#prepend!` method was doing previously, so it's deprecated from
now on.
DRY AS::SafeBuffer a bit using existing helper
Merge pull request #14529 from rwz/master

ActiveSupport::SafeBuffer#prepend inconsistency
Use asert_deprecated