Save a string allocation for each attribute method call

[casperisfine]

I noticed that the attribute method proxy call would systematically instantiate the same string over and over:

[10] pry(main)> MemoryProfiler.report { s.name? }.pretty_print
# ...
Allocated String Report
-----------------------------------
         1  "name"
         1  ~/gems/rails-1d1991430dc5/activemodel/lib/active_model/attribute_methods.rb:380

It's not a lot per say, but it can easily add up.

Backward compatibility

It's possible that there are attribute methods out there that mutate the received attribute name, but IMHO it's quite an edge case and easy to fix.

cc @rafaelfranca @Edouard-chin

[tenderlove]

ooooo lol, I love it.