Fix type_cast_for_write on Rails 4.

Numeric type_cast_for_write also needs to be patched on Rail 4 (similar
to how convert_number_column_value is handled on Rails 3.x). This fixes
AR#validates_numericality_of which uses a before_type_cast accessor.
Merge pull request #1 from thedarkone/rails-4

Merged @thedarkone pull request: Fix type_cast_for_write on Rails 4.