"./tests/test_helper.rb" and "tests/test_helper.rb" in `s.files`
are same.

This constant isn't used, so lets remove it.

The global can go bad if the compactor runs, so we need to look up the
constant instead of caching it in a global.

https://github.com/flori/json/commit/0e99a9aac5

In where to convert Hash key to String for json, this patch will add shortcut for String/Symbol in Hash key.

```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    65.000  i/100ms
Calculating -------------------------------------
                json    659.576  (± 1.5%) i/s -      3.315k in   5.027127s
```

```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    78.000  i/100ms
Calculating -------------------------------------
                json    789.781  (± 2.7%) i/s -      3.978k in   5.041043s
```

```
require 'json'
require 'benchmark/ips'

obj = []

1000.times do |i|
  obj << {
    "id" => i,
    :age => 42,
  }
end

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

https://github.com/flori/json/commit/38c0f6dbe4

To convert Hash convert, this part was using following pseudo code

```
obj.keys.each do |key|
  value = obj[key]
  ...
end
```

and `rb_funcall()` was called for `obj.keys`.
It might be slightly heavy to call the Ruby method.
This patch will iterate to convert Hash object about key/value using `rb_hash_foreach()` Ruby API instead of `rb_funcall()`.

```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    55.000  i/100ms
Calculating -------------------------------------
                json    558.501  (± 1.1%) i/s -      2.805k in   5.022986s
```

```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    65.000  i/100ms
Calculating -------------------------------------
                json    659.576  (± 1.5%) i/s -      3.315k in   5.027127s
```

```
require 'json'
require 'benchmark/ips'

obj = []

1000.times do |i|
  obj << {
    "id" => i,
    :age => 42,
  }
end

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

https://github.com/flori/json/commit/a73323dc5e

flori/json@c34d01ff6a18dac04a90b2e0f820cdb1d5c7e1b2 does not
consider US-ASCII compatible but non-UTF-8 encodings, and causes
an error in RDoc tests.

https://github.com/flori/json/commit/4f471bf590

## Before
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json   129.000  i/100ms
Calculating -------------------------------------
                json      1.300k (± 2.3%) i/s -      6.579k in   5.064656s
```

## After
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json   189.000  i/100ms
Calculating -------------------------------------
                json      1.964k (± 3.3%) i/s -      9.828k in   5.011237s
```

## Code
```
require 'json'
require 'benchmark/ips'

obj = []

1000.times do |i|
  obj << {
    "id" => i,
    :age => 42,
  }
end

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

https://github.com/flori/json/commit/c34d01ff6a

`rb_funcall` might be slightly heavy to call the Ruby method.
This patch will convert String encoding using `rb_str_encode()` instead of `rb_funcall()`.

## Before
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    78.000  i/100ms
Calculating -------------------------------------
                json    789.781  (± 2.7%) i/s -      3.978k in   5.041043s
```

## After
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json   129.000  i/100ms
Calculating -------------------------------------
                json      1.300k (± 2.3%) i/s -      6.579k in   5.064656s
```

## Code
```
require 'json'
require 'benchmark/ips'

obj = []

1000.times do |i|
  obj << {
    "id" => i,
    :age => 42,
  }
end

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

https://github.com/flori/json/commit/9ae6d2969c