Fix inclusion of excluded file types

[dmitrivereshchagin]

It may be convenient to exclude a file type from the .ackrc file. However, file types defined with the --type-set option and file types with multiple filters cannot be re-enabled from the command line.

[n1vux]

Thank you for including a test, and fixing a XXX FixMe.

Test passes, change looks reasonable.

The anticipated, general solution to a project or home-dir .ackrc sometimes being overkill / doing the wrong thing for a current desire is to use the --noenv option to ignore both .ackrc and $ACK_VARIABLES_IN_ENV, falling back to defaults, and then you can use --type theOneIgnoredInAckrc.

(Alternatively, you can comment out that line in .ackrc for an hour with a leading # and then un-comment it when done. I'd leave the editor open to remind me to undo it!)