Allow override_options to set an option to undef to completely remove it

allowing reverting to mysql's default value.

Note that previously a value of :undef would have led to a nonsensical
my.cnf file.

Author: radford

README.md

@@ -82,8 +82,7 @@ For items that you would traditionally represent as:
 thing = X
 </pre>
 
-You can just make an entry like `thing => true`, `thing => value`, or `thing => "` in the hash. You can also pass an array `thing => ['value', 'value2']` or even list each `thing => value` separately on separate lines. MySQL doesn't care if 'thing' is alone or set to a value; it'll happily accept both.
-
+You can just make an entry like `thing => true`, `thing => value`, or `thing => "` in the hash. You can also pass an array `thing => ['value', 'value2']` or even list each `thing => value` separately on separate lines. MySQL doesn't care if 'thing' is alone or set to a value; it'll happily accept both.  To keep an option out of the my.cnf file, e.g. when using override_options to revert to a default value, you can pass thing => undef.
 If an option needs multiple instances, you can pass an array. For example
 
 ```puppet
@@ -164,7 +163,7 @@ For items that you would traditionally represent as:
 thing = X
 </pre>
 
-You can just make an entry like `thing => true`, `thing => value`, or `thing => "` in the hash. You can also pass an array `thing => ['value', 'value2']` or even list each `thing => value` separately on separate lines. MySQL doesn't care if 'thing' is alone or set to a value; it'll happily accept both.
+You can just make an entry like `thing => true`, `thing => value`, or `thing => "` in the hash. You can also pass an array `thing => ['value', 'value2']` or even list each `thing => value` separately on separate lines. MySQL doesn't care if 'thing' is alone or set to a value; it'll happily accept both.  To keep an option out of the my.cnf file, e.g. when using override_options to revert to a default value, you can pass thing => undef.
 
 #####`config_file`
 

templates/my.cnf.erb

@@ -8,7 +8,7 @@
 <%         vi.each do |vii| -%>
 <%=          ki %> = <%= vii %>
 <%         end -%>
-<%       else -%>
+<%       elsif vi != :undef -%>
 <%=        ki %> = <%= vi %>
 <%       end -%>
 <%     end -%>