New OT: Split x merge simple case creates different content for clients

[scofalik]

There is a problem in this scenario, where one user merges at {} and the other splits at []:

<heading>Foo</heading>{}<paragraph>B[]ar</paragraph>

After merge It becomes:

<heading>FooB[]ar</heading>

Then split at [] creates another heading:

<heading>FooB</heading><heading>ar</heading>

While on the other client there is:

<heading>Foo</heading>{}<paragraph>B</paragraph><paragraph>ar</paragraph>

And merge leaves us with paragraph:

<heading>FooB</heading><paragraph>ar</paragraph>