Does not check whether illegal utf-8 if string has ascii only.

## Before
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    25.000  i/100ms
Calculating -------------------------------------
                json    250.478  (± 4.8%) i/s -      1.250k in   5.002238s
```

## After
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    32.000  i/100ms
Calculating -------------------------------------
                json    360.652  (± 3.6%) i/s -      1.824k in   5.064511s
```

## Test code
```
require 'json'
require 'benchmark/ips'

obj = []

1000.times do |i|
  obj << {
    :string => "x" * 100,
    :utf8 => "あ" * 100
  }
end

Benchmark.ips do |x|
  x.report "json" do |iter|
    count = 0
    while count < iter
      JSON.generate(obj)
      count += 1
    end
  end
end
```

0 files changed, 0 insertions, 0 deletions