Fix: allow Timber\PostExcerpt::read_more to accept bool value

[Levdbas]

Related:

• Allow the Timber\PostPreview::read_more to accept a boolean value #2578

Issue

The [Timber\PostPreview::read_more](https://timber.github.io/docs/reference/timber-postpreview/#read_more) docs say:

    Set this to false to not add a "Read More" link.

But Timber\PostPreview::read_more doesn't actually accept a boolean value, it only accepts a string value type. 😬

Solution

Port changes of #2578 to v2.

Impact

Usage Changes

No

Considerations

Testing

Already covered by tests in TestTimberPostExcerptObject::testExcerptConstructorWithWords

[coveralls]

coverage: 87.745% (-0.06%) from 87.807%
when pulling 894e214 on postexcert-readmore
into 13c6b0f on 2.x.