Improve Plain Style support

[Himura2la]

The test in #521 is invalid, and did not test the real quotation cases.

I fixed the test and changed the isPlainSafe function so that it follows the specs more precisely.

Although, I still don't understand why specs exclude s-white from ns-plain-char:

[34]            ns-char ::= nb-char - s-white
[127]  ns-plain-safe(c) ::= c = flow-out ⇒ ns-plain-safe-out
                            ...
[128] ns-plain-safe-out ::= ns-char
[130]  ns-plain-char(c) ::= ( ns-plain-safe(c) - “:” - “#” )
                            ...

ns-plain-char(flow-out) ::= nb-char - s-white - “:” - “#”

But we don't see it even in examples:

- Up, up, and away!

I included whitespaces into Plain scalars as they were before my changes

[rlidwka]

Merged into dev branch as 91f9662 (after resolving conflicts), gonna be released as 4.0 later. Thanks!