`datetime_format=` does not apply to custom formatters

[Rubyist007]

When a custom formatter that responds to datetime_format= is set on a Logger, calling Logger#datetime_format= applies the format to the internal @default_formatter instead of the active formatter. This means the datetime format has no effect on log output.

Steps to reproduce

require 'logger'

custom_formatter = Logger::Formatter.new
logger = Logger.new($stdout, formatter: custom_formatter, datetime_format: "%d%b%Y@%H:%M:%S")

# Expected: datetime_format is set on custom_formatter
logger.datetime_format # => nil (unexpected)
custom_formatter.datetime_format # => nil (unexpected)
logger.info("hello")
# Outputs default datetime format instead of "%d%b%Y@%H:%M:%S"

The same issue occurs when setting datetime_format after initialization:

logger = Logger.new($stdout, formatter: Logger::Formatter.new)
logger.datetime_format = "%d%b%Y@%H:%M:%S"
logger.info("hello")
# Still uses default datetime format

Root cause

Logger#datetime_format= always delegates to @default_formatter:

def datetime_format=(datetime_format)
 @default_formatter.datetime_format = datetime_format
end

But format_message uses @formatter when set:

def format_message(severity, datetime, progname, msg)
 (@formatter || @default_formatter).call(severity, datetime, progname, msg)
end

Additionally, in initialize, datetime_format is set before the custom formatter is assigned, so it never reaches the custom formatter even during construction.

Expected behavior

datetime_format= should apply to the active formatter (custom or default) if it responds to datetime_format=.