Avoid redundant `attribute_alias?` before `attribute_alias`

If we want to get alias resolved attribute finally, we can use
`attribute_alias` directly.

For that purpose, avoiding redundant `attribute_alias?` makes alias
attribute access 40% faster.

https://gist.github.com/kamipo/e427f080a27b46f50bc508fae3612a0e

Before (2c0729d8):

```
Warming up --------------------------------------
          user['id']   102.668k i/100ms
      user['new_id']    80.660k i/100ms
        user['name']    99.368k i/100ms
    user['new_name']    81.626k i/100ms
Calculating -------------------------------------
          user['id']      1.431M (± 4.0%) i/s -      7.187M in   5.031985s
      user['new_id']      1.042M (± 4.2%) i/s -      5.243M in   5.039858s
        user['name']      1.406M (± 5.6%) i/s -      7.055M in   5.036743s
    user['new_name']      1.074M (± 3.6%) i/s -      5.387M in   5.024152s
```

After (this change):

```
Warming up --------------------------------------
          user['id']   109.775k i/100ms
      user['new_id']   103.303k i/100ms
        user['name']   105.988k i/100ms
    user['new_name']    99.618k i/100ms
Calculating -------------------------------------
          user['id']      1.520M (± 6.7%) i/s -      7.574M in   5.011496s
      user['new_id']      1.485M (± 6.2%) i/s -      7.438M in   5.036252s
        user['name']      1.538M (± 5.4%) i/s -      7.737M in   5.049765s
    user['new_name']      1.516M (± 4.6%) i/s -      7.571M in   5.007293s
```

1 files changed, 5 insertions, 10 deletions

diff --git a/activemodel/lib/active_model/attributes.rb b/activemodel/lib/active_model/attributes.rb
index 6048ff0113..d176ea88d0 100644
--- a/activemodel/lib/active_model/attributes.rb
+++ b/activemodel/lib/active_model/attributes.rb
@@ -116,22 +116,17 @@ module ActiveModel
     private
 
       def write_attribute(attr_name, value)
-        name = if self.class.attribute_alias?(attr_name)
-          self.class.attribute_alias(attr_name).to_s
-        else
-          attr_name.to_s
-        end
+        name = attr_name.to_s
+        name = self.class.attribute_aliases[name] || name
 
         @attributes.write_from_user(name, value)
         value
       end
 
       def attribute(attr_name)
-        name = if self.class.attribute_alias?(attr_name)
-          self.class.attribute_alias(attr_name).to_s
-        else
-          attr_name.to_s
-        end
+        name = attr_name.to_s
+        name = self.class.attribute_aliases[name] || name
+
         @attributes.fetch_value(name)
       end