Fix no help text showing when option's default is NA

Hi Trevor,

Thank you for great port of optparse to R! I've realized today that when defining an option with a default value of `NA`, the help message printed by the `-h` switch will not show a help text for this option. Consider e.g. the following minimal example:

    #!/usr/bin/env Rscript
    library(optparse)
    
    option_list <- list(
    	make_option(c("-o", "--long-option"), dest="my_option", default=NA, help="An option [default: %default]")
    )
    opt <- parse_args(OptionParser(option_list = option_list))
    
    print(opt)

Running this from the command line I get:

    $ ./example.R -h
    Usage: ./example.R [options]
    
    
    Options:
    	-o LONG-OPTION, --long-option=LONG-OPTION
    		NA
    
    	-h, --help
    		Show this help message and exit


I've traced this to the [sub call](https://github.com/trevorld/optparse/blob/master/R/optparse.R#L265) for replacing the `%default` variable in the help text. When the replacement is `NA`, the whole returned string will be `NA` (the constant, not the string!). The replacement is `NA` because `as.character(NA) = NA`, not `"NA"` as expected.

The pull request adds a check for `NA` in the `default_str` to check for this corner case and replace `NA` with `"NA"` so that the help text can be correctly formatted.

Best,
Stefan