[ruby/logger] Changes to datetime formatting

Formatting a datetime should only pertain to itself and valid datetimes do not contain a space. Should there be a desire to show show a space between the datetime and the process pid in the formatted log, this formatting logic should take place there.
Furthermore, the default datetime format is moved to a class variable to allowing this variable to be overwritten by subclasses.

https://github.com/ruby/logger/commit/7cbd434349

lib/logger/formatter.rb

@@ -3,7 +3,8 @@
 class Logger
   # Default formatter for log messages.
   class Formatter
-    Format = "%s, [%s#%d] %5s -- %s: %s\n"
+    Format = "%s, [%s #%d] %5s -- %s: %s\n"
+    DatetimeFormat = "%Y-%m-%dT%H:%M:%S.%6N"
 
     attr_accessor :datetime_format
 
@@ -19,7 +20,7 @@ class Logger
   private
 
     def format_datetime(time)
-      time.strftime(@datetime_format || "%Y-%m-%dT%H:%M:%S.%6N ")
+      time.strftime(@datetime_format || DatetimeFormat)
     end
 
     def msg2str(msg)