Optimize Sanitize#transform_node!
Optimize Sanitize#transform_node!

Since transform_node! may be called in a tight loop to process thousands
of items, we can optimize both memory and CPU performance by:

1. Reusing the same config hash for each transformer
2. Directly assigning values to hash instead of using merge!. Not only does
merge! create a new hash, it is also 2.6x slower:
https://github.com/JuanitoFatas/fast-ruby#hashmerge-vs-hash-code

rgrove/sanitize,rgrove/sanitize