Anytime `ActionDispatch::HTTP::Reponse#content_type=` is called, the
`content_type` argument is parsed using the CONTENT_TYPE_PARSER regexp.
If the `content_type` argument comes from `Mime`, there is no need to
parse it using the regexp as we already know the result.

```ruby
require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "rails"
  gem "benchmark-ips"
end

require "action_controller/railtie"
require 'benchmark/ips'

Benchmark.ips do |x|
  x.config(:time => 5, :warmup => 2)

  response = ActionDispatch::Response.new
  x.report("main") do
    response.content_type = Mime[:html].to_s
  end

  class BranchResponse < ActionDispatch::Response
    def content_type=(content_type)
      case content_type
      when NilClass
        return
      when Symbol
        new_header_info = ContentTypeHeader.new(Mime[content_type])
      else
        new_header_info = parse_content_type(content_type.to_s)
      end

      prev_header_info = parsed_content_type_header
      charset = new_header_info.charset || prev_header_info.charset
      charset ||= self.class.default_charset unless prev_header_info.mime_type
      set_content_type new_header_info.mime_type, charset
    end
  end

  response = BranchResponse.new
  x.report("branch") do
    response.content_type = :html
  end

  x.compare!
end
```

```
Warming up --------------------------------------
                main    38.666k i/100ms
              branch    52.235k i/100ms
Calculating -------------------------------------
                main    393.529k (± 3.3%) i/s    (2.54 μs/i) -      1.972M in   5.016529s
              branch    494.789k (± 2.2%) i/s    (2.02 μs/i) -      2.507M in   5.069774s

Comparison:
              branch:   494788.6 i/s
                main:   393529.2 i/s - 1.26x  slower
```

This improves the plain json call in the TechEmpower benchmark by
reducing the time spent in `#assign_default_content_type_and_charset!`
from 16% to 6.5%.
It probably improves other calls as well.