Ignore unset CPAN metadata fields

Otherwise this can happen:

    $ fpm -t deb -s cpan --verbose 'Lingua::JA::Romanize::Japanese'
    Asking metacpan about a module {:module=>"Lingua::JA::Romanize::Japanese", :level=>:info}
    Downloading perl module {:distribution=>"Lingua-JA-Romanize-Japanese", :version=>nil, :level=>:info}
    Setting package name from 'name' {:name=>"Lingua-JA-Romanize-Japanese", :level=>:info}
    /usr/lib/ruby/gems/1.9.1/gems/fpm-1.1.0/lib/fpm/package/cpan.rb:86:in `input': undefined method `join' for nil:NilClass (NoMethodError)
	    from /usr/lib/ruby/gems/1.9.1/gems/fpm-1.1.0/lib/fpm/command.rb:299:in `block in execute'
	    from /usr/lib/ruby/gems/1.9.1/gems/fpm-1.1.0/lib/fpm/command.rb:298:in `each'
	    from /usr/lib/ruby/gems/1.9.1/gems/fpm-1.1.0/lib/fpm/command.rb:298:in `execute'
	    from /usr/lib/ruby/gems/1.9.1/gems/clamp-0.6.3/lib/clamp/command.rb:67:in `run'
	    from /usr/lib/ruby/gems/1.9.1/gems/fpm-1.1.0/lib/fpm/command.rb:449:in `run'
	    from /usr/lib/ruby/gems/1.9.1/gems/clamp-0.6.3/lib/clamp/command.rb:125:in `run'
	    from /usr/lib/ruby/gems/1.9.1/gems/fpm-1.1.0/bin/fpm:8:in `<top (required)>'
	    from /usr/bin/fpm:23:in `load'
	    from /usr/bin/fpm:23:in `<main>'
Add --rpm-verbatim-gem-dependencies flag

This should help folks who want the old-style gem-rpm conversions
* with this flag, old behavior: rubygem-name
* without flag, default behavior: rubygem(name)

The 'default' behavior has been this behavior since 0.4.23 for most
users.
If you changed the package name prefix via --gem-package-prefix or
--gem-package-name-prefix then this behavior was broken.
It was fixed in #585.

But, I don't want folks relying on old behavior to be screwed
when that behavior mysteriously vanishes! This should be a good
enough work around for now.

Reported by Ryan Doherty via the fpm-users mailing list.
Merge pull request #724 from jordansissel/new-flag/rpm-verbatim-gem-dependencies

Add --rpm-verbatim-gem-dependencies flag
Merge pull request #712 from mlafeldt/cpan-metadata-nil

Ignore unset CPAN metadata fields