Improve MINIMIZE_QUOTES handling to avoid quoting for some use of # and :

[frantuma]

#181 and 2 more #180 related commits introduce some improved YAML scalars quoting more compliant to the YAML Spec, in the MINIMIZE_QUOTES mode; the updated behavior seems to be at least more strict (in quoting) than necessary, as it introduces quoting for any value containing chars as # and :.

If I am not mistaken YAML Spec states that such plain style limitation is applied for cases where the character combinations # (<space_or_tab>+<#>) and : (<:>+<space_or_tab>) are present, not the single chars # and :.

This PR updates behavior by considering the character combinations and not the single chars.

This has been triggered by an update of Jackson from 2.10.4 to 2.11.0 which caused YAML serialization test failures due to the difference in quoting, e.g. in case of scalar representing URIs, JSON references, date or datetime.

I understand that this is probably not a major issue, and e.g. code or tests can be adjusted in dependent projects and downstream, but I believe the affected scenarios are quite common and it would make sense to keep quotes at a minimum when minimize is on.

Not sure if the target branch 2.11 is the correct one, was wondering if this could make it for 2.11.1 patch release in case.

[cowtowncoder]

@frantuma Thank you! I think you nailed it; I like the idea of more targeted quoting, and I think risk/reward ratio works for 2.11.1 (early enough patch). Will merge.