Previously, if two attributes in t-att-class shared some classes, their
presence would be determined by the last attribute declared, instead of
being present if any attribute containing it evaluates to true. This
commit fixes that.
the low level method htmlelement.classList.add does not accept multiple
classes in one string, which is why, in owl, the expression
`<div t-att-class="{'a b c': value}" />`
did not work as one might expect. It is however very convenient in real
life templates, so this commit improve owl by adding support for this
feature.
closes#813
Géry Debongnie