Refactor Hash#transform_values and Hash#transform_values!

This patch makes two small changes to the new Active Support core extensions `Hash#transform_values` and `Hash#transform_values!`:

1. These methods now return an `Enumerator` if no block is given. This makes them more consistent with other `Enumerable` methods and allows for method chaining, for example:

    ```ruby
    {a: 1, b: 2, c: 3}.transform_values.with_index{|x, i| [x * x, i]}
    #=> {:a=>[1, 0], :b=>[4, 1], :c=>[9, 2]}
    ```

    Before this patch, such chaining would result in a `LocalJumpError: no block given`.

2. Since the `&block` parameter is never referenced, I’ve removed it from `Hash#transform_values`. This prevents a garbage Proc object from being constructed automatically on method invocation, resulting in measurably faster performance. Here is the benchmark I ran:

    ```ruby
    require 'benchmark/ips'

    class Hash
      def slow(&block)
        return enum_for(:slow) unless block_given?
        result = self.class.new
        each do |key, value|
          result[key] = yield(value)
        end
        result
      end

      def fast
        return enum_for(:fast) unless block_given?
        result = self.class.new
        each do |key, value|
          result[key] = yield(value)
        end
        result
      end
    end

    HASH = {a: 1, b: 2, c: 3}

    Benchmark.ips do |x|
      x.report("slow") { HASH.slow { |x| x * x } }
      x.report("fast") { HASH.fast { |x| x * x } }
    end
    ```

    ```
    slow 388660.2 (±10.6%) i/s - 1931275 in 5.030443s
    fast 621865.9 (±11.5%) i/s - 3085425 in 5.029155s
    ```

References https://github.com/rails/rails/pull/15819. cc/ @guilleiguaran @rafaelfranca @sgrif
I was more just expressing subtle surprise that we didn't already have a test that expected HWIA#transform_keys to return a plain Hash. I have no actual objection to the more consistent behaviour.