Convert String encoding using rb_str_encode()

`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
```

Author: Watson1978

ext/json/ext/generator/generator.c

@@ -846,7 +846,7 @@ static void generate_json_string(FBuffer *buffer, VALUE Vstate, JSON_Generator_S
 {
     fbuffer_append_char(buffer, '"');
 #ifdef HAVE_RUBY_ENCODING_H
-    obj = rb_funcall(obj, i_encode, 1, CEncoding_UTF_8);
+    obj = rb_str_encode(obj, CEncoding_UTF_8, 0, Qnil);
 #endif
     if (state->ascii_only) {
         convert_UTF8_to_JSON_ASCII(buffer, obj);