Performance regression (2.4->2.5), 10% slower write via databind

(note: most like this is from #126)

Running `jackson-benchmarks` on 2.5.0, I noticed that XML serialization is 10-15% slower than with 2.4.
I suspect this is due to inefficient checks done for a new feature: instead of adding information to be efficiently accessible (similar to how 'is-attribute' is handled), a dynamic lookup is done for each and write write of a value. This should be changed.
