Even that collisions are unlikely we need to make sure the two strings
are equal.
Timing is not important in this case because this only runs after the
comparison between the SHA256 digested strings returns true.
I got this error in production using Puma in multi-threaded mode:
```
RuntimeError: Digest::Base cannot be directly inherited in Ruby
from active_support/security_utils.rb:23:in `variable_size_secure_compare'
from active_support/security_utils.rb:23:in `hexdigest'
from active_support/security_utils.rb:23:in `digest'
```
Looks like Digest uses const_missing to load Digest::SHA256 (https://github.com/ruby/ruby/blob/trunk/ext/digest/lib/digest.rb#L8)
- https://bugs.ruby-lang.org/issues/9494
- c02fa39463
to make it not leak length information even for variable length string.
Renamed old `ActiveSupport::SecurityUtils.secure_compare` to `fixed_length_secure_compare`,
and started raising `ArgumentError` in case of length mismatch of passed strings.
