allow inserting attributes that have zero length

Otherwise the strlen() might give an incorrect length if the attribute
value is just a part of a longer string.

Change-Id: I67eb7baecfa928fdee26c5ea9003bd7fc9b96d59
make FastAttributeList's unknown attribute value be passed as OString

Since that's how it's internally stored anyway, and I have a use case
where it's useful to limit the length by passing it to OString ctor.

Change-Id: I5903ea4f1b2cdb48c1bbceac6b8e21eb5882d377