Remove useless `coerce` call to optimize enum type
Remove useless `coerce` call to optimize enum type

`coerce` method is called once wastefully in enum type.
This change removes useless call.

Benchmark
=======

```ruby
 # bench.rb
require 'benchmark'
require 'strong_json'

s = StrongJSON.new do
  let :test, enum(string, number, boolean)
end

Benchmark.bm(20) do |x|
  x.report('string') {1000000.times{s.test.coerce('foo')}}
  x.report('number') {1000000.times{s.test.coerce(42)}}
  x.report('boolean'){1000000.times{s.test.coerce(true)}}
end
```

```bash
 # before
$ ruby bench.rb
                           user     system      total        real
string                 1.000000   0.000000   1.000000 (  1.000055)
number                 2.660000   0.000000   2.660000 (  2.666975)
boolean                4.240000   0.000000   4.240000 (  4.239681)

 # after
$ ruby bench.rb
                           user     system      total        real
string                 0.680000   0.000000   0.680000 (  0.681966)
number                 2.480000   0.000000   2.480000 (  2.482622)
boolean                4.150000   0.000000   4.150000 (  4.143135)
```

soutaro/strong_json