function defaultAttrConcat(attrValue, attrSpaces) {
	  return "" + attrSpaces.before + attrValue + attrSpaces.after;
	}
Set the case insensitive flag.
If the case insensitive flag changes, the raw (escaped) value at `attr.raws.insensitiveFlag`
of the attribute is updated accordingly.

@param {true | false} insensitive true if the attribute should match case-insensitively.