Use rb_enc_get() to retrieve encoding type

[Watson1978]

To reduce overhead, this patch use rb_enc_get() instead.

−	before	after	result
Oj.dump	1.319M	1.336M	-
Oj.dump (compat)	948.393k	1.125M	1.19x
Oj.dump (rails)	815.509k	943.996k	1.16x

Environment

• MacBook Air (M1, 2020)
• macOS 12.0 beta 4
• Apple M1
• Ruby 3.0.2

Before

Warming up --------------------------------------
             Oj.dump   131.400k i/100ms
    Oj.dump (compat)    94.239k i/100ms
     Oj.dump (rails)    79.788k i/100ms
Calculating -------------------------------------
             Oj.dump      1.319M (± 0.7%) i/s -      6.701M in   5.080504s
    Oj.dump (compat)    948.393k (± 0.9%) i/s -      4.806M in   5.068098s
     Oj.dump (rails)    815.509k (± 0.8%) i/s -      4.149M in   5.087955s

After

Warming up --------------------------------------
             Oj.dump   133.596k i/100ms
    Oj.dump (compat)   113.563k i/100ms
     Oj.dump (rails)    95.233k i/100ms
Calculating -------------------------------------
             Oj.dump      1.336M (± 1.0%) i/s -      6.680M in   5.000748s
    Oj.dump (compat)      1.125M (± 0.9%) i/s -      5.678M in   5.046279s
     Oj.dump (rails)    943.996k (± 0.8%) i/s -      4.762M in   5.044464s

Test code

require 'benchmark/ips'
require 'oj'

json =<<-EOF
{
  "$id": "https://example.com/person.schema.json",
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "Person",
  "type": "object",
  "properties": {
    "firstName": {
      "type": "string",
      "description": "The person's first name."
    },
    "lastName": {
      "type": "string",
      "description": "The person's last name."
    },
    "age": {
      "description": "Age in years which must be equal to or greater than zero.",
      "type": "integer",
      "minimum": 0
    }
  }
}
EOF

Benchmark.ips do |x|
  data = Oj.load(json)
  x.report('Oj.dump') { Oj.dump(data) }
  x.report('Oj.dump (compat)') { Oj.dump(data, mode: :compat) }
  x.report('Oj.dump (rails)') { Oj.dump(data, mode: :rails) }
end

[Watson1978]

I found the same code in another place.

oj/ext/oj/parse.c

Lines 966 to 971 in 4fdd25d

	static void oj_pi_set_input_str(ParseInfo pi, volatile VALUE *inputp) {
	rb_encoding *enc = rb_to_encoding(rb_obj_encoding(*inputp));
	if (rb_utf8_encoding() != enc) {
	*inputp = rb_str_conv_enc(*inputp, enc, rb_utf8_encoding());
	}

[Watson1978]

OK, I updated