Improve performance for serialization of entities

Hi there,

I spent a little bit of time profiling grape-entity using [ruby-prof](https://github.com/ruby-prof/ruby-prof), and found a couple of ways to improve performance during the serialization process. This changeset sees Grape::Entity serializing ~2.5 times faster, by my benchmark. 

Before:
```
$ bundle exec ruby bench/serializing.rb
       user     system      total        real
serializing  1.800000   0.000000   1.800000 (  1.800179)
```

After:
```
$ bundle exec ruby bench/serializing.rb
       user     system      total        real
serializing  0.750000   0.000000   0.750000 (  0.751563)
```