Spring Reduce allocations caused by producible media types

Hi,

this PR reduces some (repeated) allocations of LinkedHashSets and corresponding Nodes caused by ProducesRequestCondition.getProducibleMediaTypes().

I noticed this in a fairly vanilla app where this causes somewhat around 2-3% of the allocations during runtime, so not too huge, but noticeable.

Cheers, Christoph

Comment From: sdeleuze

Merged, thanks!