Fix spellcheck attr not set to false

[marvinhagemeister]

I'm not too keen on checking directly against the attribute name, but this seems to be the only case in the spec where the attribute should not be removed if the value is false.

const el = document.createElement("input");

el.removeAttribute("spellCheck");
el.spellcheck // logs `true` :/

el.setAttribute("spellCheck", false);
el.spellcheck // logs `false`

Fixes #651.

[coveralls]

Coverage remained the same at 100.0% when pulling 836c99d on marvinhagemeister:spellcheck into 7e49a91 on developit:master.

[marvinhagemeister]

Locally, without code-coverage enabled the performance difference is much smaller than in our CI:

test	before	after
empty diff	87939/s (30 ticks)	88119/s (31 ticks)
repeat diff	10988/s (250 ticks)	11709/s (228 ticks)
large VTree	43701/s (63 ticks)	43082/s (62 ticks)
style/prop mutation	20460/s (134 ticks)	20316/s (133 ticks)
SSR Hydrate	1376/s (1967 ticks)	1315/s (2020 ticks)

[developit]

I think the name is "spellcheck", right? In that case it actually wouldn't fall through to here, but rather would pass the in check on line 82. It seems like the special-casing belongs there:

// replaces L84:
if ((value==null || value===false) && name!='spellcheck') node.removeAttribute(name);

[marvinhagemeister]

Went the setAttribute way, but forcing lowercase seems better. Will change that.

[marvinhagemeister]

@developit Updated with your suggestion 🎉