Optimize Action Text's plain text conversion

[javan]

Fixes that converting deeply nested elements could exceed the stack level, as seen in the added test failing before the optimization:

.....................................E

Error:
ActionText::PlainTextConversionTest#test_deeply_nested_tags_are_converted:
SystemStackError: stack level too deep

See also: https://twitter.com/javan/status/1164545630079016965

[kamipo]

Is not compact necessary? (plain_text_for_node will not return nil?)

[javan]

It doesn't appear to be necessary, and I don't see nil being returned anywhere. I don't think it's an issue either way since nils would disappear as empty strings when the array is joined:

>> [ "a", nil, nil, "b", "", nil, "c" ].join("")
=> "abc"