fix: can't modify frozen String: "decimal" (#895)

Resolves: #834

Author: n-rodriguez

lib/annotate/annotate_models.rb

@@ -239,7 +239,7 @@ def get_col_type(col)
         'bigint'
       else
         (col.type || col.sql_type).to_s
-      end
+      end.dup
     end
 
     def index_columns_info(index)