The example in documentation states:

> Specified as comma-separated key:value pairs, e.g. "SERVICE_REALM:foo,CANONICALIZE_HOSTNAME:TRUE".

Passing this input (with an extra underscore per https://jira.mongodb.org/browse/RUBY-1757) produces:

irb(main):007:0> Mongo::URI.new('mongodb://test.host/?authMechanismProperties=SERVICE_REALM:foo,CANONICALIZE_HOST_NAME:TRUE').uri_options[:auth_mech_properties]
=> {"service_realm"=>"foo", "canonicalize_host_name"=>false}

This is because the code only accepts `true` as the true value and converts everything else to false.

At a minimum, TRUE should be converted to true, and possibly other values like 1 should also be converted to true.