setScalarValue reconstructed a block scalar's content indent as token.indent
plus one step, but token.indent is the indent of the node, not of its
contents. Contents sitting further in than one step were pulled back, and
without afterKey they lost their indentation entirely, which for a block
scalar in a collection changes what the document means rather than just how
it looks.
resolveBlockScalar already measures the real content indent when reading: an
explicit indentation indicator counts from the node indent, and otherwise the
indent is that of the first non-empty line. Measure the same way before
stringifying, and fall back to the previous behaviour when a block scalar has
no contents to measure.
Fixes#349
dualfroz
upstream