(#16922) Quote strings that contain ":"

Part of the performance work on the ZAML code caused a regression on how
to convert values that contain a colon into YAML strings. This becomes a
problem when a string ends in a colon and that line in the YAML output
is followed by another line.

An example of the incorrect output is:

a: a:
b: 1

This patch causes that to be output correctly as:

a: "a:"
b: 1