Added keys method to Server properties

When generating servers programmatically this provides a method
of determining what properties have been added.
Includes simple spec test

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 Reverse Chronological Order:
 
+## master
+
+* Minor changes:
+  * Added `keys` method to Server properties to allow introspection of automatically added
+    properties.
+
 ## `3.1.0`
 
 Breaking changes:

lib/capistrano/configuration/server.rb

@@ -86,6 +86,10 @@ module Capistrano
           @roles ||= Set.new
         end
 
+        def keys
+          @properties.keys
+        end
+
         def method_missing(key, value=nil)
           if value
             set(lvalue(key), value)

spec/lib/capistrano/configuration/servers_spec.rb

@@ -104,6 +104,7 @@ module Capistrano
           expect(servers.roles_for([:app]).first.hostname).to eq '1'
           expect(servers.roles_for([:web]).first.hostname).to eq '1'
           expect(servers.roles_for([:all]).first.properties.test).to eq :value
+          expect(servers.roles_for([:all]).first.properties.keys).to eq [:test]
         end
 
         it 'can accept multiple servers with the same hostname but different ports or users' do