Speedup encodable check

This is showing up as the most expensive line in my CPU profile of the Riak client. Pretty simple fix, appears compatible with all Ruby versions.

Timing in milliseconds on 10,000 iterations:
```ruby
#public_method_defined?
{:real=>0.944, :cpu=>1, :idle=>-0.05600000000000005}

#public_instance_methods.include?
{:real=>1202.375, :cpu=>1194, :idle=>8.375}
```
https://gist.github.com/eac/5011b6c53578c1d20773