Escape structural characters in keys when writing

[gaoflow]

Write escaped only space and : in a key, but the lexer treats two more characters as structural in a key position (lex.go): = ends a key (isEndOfKey) and a leading #/! starts a comment (isComment). So Load(Write(p)) was not identity for such keys — = split the key and a leading #/! dropped the whole line.

p := properties.NewProperties()
p.MustSet("a=b", "c")
var b bytes.Buffer
p.Write(&b, properties.UTF8)            // a=b = c
p2, _ := properties.LoadString(b.String())
_, ok := p2.Get("a=b")                 // ok == false, key parsed as "a"

Escaping = and a leading #/! fixes it; the parser already reads \=, \#, \!. Mid-key #/! are left unescaped since they are only structural at line start (and already parse fine, e.g. key#2 in the read tests). The value path is unchanged — none of these are structural after the separator.

Added round-trip and exact-output tests covering every special key character in both encodings. Full suite, gofmt and go vet pass.

Fixes #59