Test command name validation and fix up some bugs

lib/vagrant/plugin/v1.rb

@@ -73,13 +73,15 @@ module Vagrant
       def self.command(name=UNSET_VALUE, &block)
         data[:command] ||= Registry.new
 
+        if name != UNSET_VALUE
           # Validate the name of the command
-        if name.to_s !~ /^[-a-z0-9]/i
+          if name.to_s !~ /^[-a-z0-9]+$/i
             raise InvalidCommandName, "Commands can only contain letters, numbers, and hyphens"
           end
 
           # Register a new command class only if a name was given.
-        data[:command].register(name.to_sym, &block) if name != UNSET_VALUE
+          data[:command].register(name.to_sym, &block)
+        end
 
         # Return the registry
         data[:command]

test/unit/vagrant/plugin/v1_test.rb

@@ -44,6 +44,15 @@ describe Vagrant::Plugin::V1 do
       plugin.command[:foo].should == "bar"
     end
 
+    ["spaces bad", "sym^bols"].each do |bad|
+      it "should not allow bad command name: #{bad}" do
+        plugin = Class.new(described_class)
+
+        expect { plugin.command(bad) {} }.
+          to raise_error(Vagrant::Plugin::V1::InvalidCommandName)
+      end
+    end
+
     it "should lazily register command classes" do
       # Below would raise an error if the value of the command class was
       # evaluated immediately. By asserting that this does not raise an