This reverts commit abf8de85.
We should take a deeper look to those cases flat_map doesn't do deep
flattening.

irb(main):002:0> [[[1,3], [1,2]]].map{|i| i}.flatten
=> [1, 3, 1, 2]
irb(main):003:0> [[[1,3], [1,2]]].flat_map{|i| i}
=> [[1, 3], [1, 2]]

In Textile `*` would convert to `<strong>`, but in Markdown we have to
use `**` instead.

This functionality will be available from gem
`active_record-session_store` instead.

Add Hash#deep_transform_keys and Hash#deep_transform_keys! Also convert deep_*_keys to use deep_transform_keys.

[ci skip]

<plus> is nonstandard Textile; RailsGuides::TextileExtensions::plusplus
is the method that causes multiple code-formatted `+` symbols to
generate crap like

    <code>1 <notextile><tt>+</tt></notextile> 2 ...</code>

instead of the expected output:

    <code>1 + 2 ...</code>

Remove accidentally pasted NOTE section during introduction of +beginning_of_hour+ and +end_of_hour+ documentation.

Added documentation for +beginning_of_hour+ and +end_of_hour+ to Active Support Core Extensions guide.