New characters escaped in serialize_name

[mskrzypkows]

Added escape of NULL (U+0000) char and escape for characters
in the range [\1-\1f](U+0001 to U+001F) or U+007F
according to https://drafts.csswg.org/cssom/#serialize-an-identifier
Needed for proper CSS tests servo/servo#10685

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This change is

[nox]

The code looks correct, but could you add some unit tests please?

[mskrzypkows]

OK I'll add, forgot about it.

[SimonSapin]

Please add unit tests in src/tests.rs of the form:

#[test]
fn identifier_serialization() {
    assert_eq!(Token::Ident("\0").to_css_string(), "\u{FFFD}");
    assert_eq!(Token::Ident("a\0").to_css_string(), "a\u{FFFD}");
    // ...
}

with one assert_eq! for each assert_equals in https://github.com/servo/servo/blob/master/tests/wpt/css-tests/cssom-1_dev/html/escape.htm, except for "Number of arguments", "String conversion", and "Surrogates" ones. (But add one with "\u{1D306}" instead of surrogates.)

[mskrzypkows]

I've added tests.

[SimonSapin]

Great, thanks!

@bors-servo r+

[bors-servo]

📌 Commit 2f48ad9 has been approved by SimonSapin

[bors-servo]

⌛ Testing commit 2f48ad9 with merge 713c762...

[bors-servo]

☀️ Test successful - travis