if FROM is a string, convert to string literal

lib/arel/select_manager.rb

@@ -51,6 +51,7 @@ def group *columns
     end
 
     def from table
+      table = Nodes::SqlLiteral.new(table) if String === table
       @ctx.froms = [table]
       self
     end

spec/arel/select_manager_spec.rb

@@ -60,7 +60,7 @@ def execute sql, name = nil
           manager.from table
           manager.from 'users'
           manager.project table['id']
-          manager.to_sql.should be_like 'SELECT "users"."id" FROM \'users\''
+          manager.to_sql.should be_like 'SELECT "users"."id" FROM users'
         end
       end