Remove unnecessary array copy and resizing for headers

JFR shows that for each header, an empty `ArrayList` is created and then a new element is added to it. This causes the code to go through `grow` unnecessarily. In addition, most of the time, it just needs 1 element for the array, while `ArrayList` creates a size 10 array by default.

It is a minor thing, but just wanted to address. Please see PR https://github.com/resteasy/Resteasy/pull/2141.