Improve the performance of writing attributes

Using a similar approach to 08576b94ad4f19dfc368619d7751e211d23dcad8,
this change adds a new internal `_write_attribute` method which bypasses
the code that checks for attribute aliases and custom primary keys.

We can use this method instead of `write_attribute` when we know that we
have the name of the actual column to be updated and not an alias.

This makes writing an attribute with `attribute=` about 18% faster.

Benchmark:

```
begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"
  gem "rails", github: "rails/rails"
  gem "arel", github: "rails/arel"
  gem "sqlite3"
  gem "benchmark-ips"
end

require "active_record"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end
end

class Post < ActiveRecord::Base
end

post = Post.new(id: 1)

Benchmark.ips do |x|
  x.report("attribute=") { post.id = post.id + 1 }
end
```

Before:

    Warming up --------------------------------------
              attribute=    25.889k i/100ms
    Calculating -------------------------------------
              attribute=    290.946k (± 3.1%) i/s -      1.476M in   5.077036s

After:

    Warming up --------------------------------------
              attribute=    30.056k i/100ms
    Calculating -------------------------------------
              attribute=    345.088k (± 4.8%) i/s -      1.743M in   5.064264s

1 files changed, 1 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/inheritance.rb b/activerecord/lib/active_record/inheritance.rb
index 5776807507..1753322274 100644
--- a/activerecord/lib/active_record/inheritance.rb
+++ b/activerecord/lib/active_record/inheritance.rb
@@ -245,7 +245,7 @@ module ActiveRecord
       def ensure_proper_type
         klass = self.class
         if klass.finder_needs_type_condition?
-          write_attribute(klass.inheritance_column, klass.sti_name)
+          _write_attribute(klass.inheritance_column, klass.sti_name)
         end
       end
   end