simplify initialization of manifest attrs

add defaults, merge with provided hash, but use default if a hash value
is  nil

lib/awesim/manifest.rb

@@ -50,14 +50,20 @@ documentation: |
       InvalidManifest.new(e)
     end
 
+    def defaults
+      {"name" => "", "provider" => "", "description" => "", "documentation" => ""}
+    end
+
     def initialize(opts)
-      raise InvalidContentError.new unless(opts && opts.respond_to?(:fetch))
+      raise InvalidContentError.new unless(opts && opts.is_a?(Hash))
+
+      # merge with defaults, ignoring nil
+      opts = defaults.merge(opts) { |key, oldval, newval| newval || oldval  }
 
-      #FIXME: add sensible default for app name?
-      @name = (opts.fetch("name", "") || "")
-      @provider = (opts.fetch("provider", "") || "")
-      @description = (opts.fetch("description", "") || "")
-      @documentation = (opts.fetch("documentation", "") || "")
+      @name = opts.fetch("name")
+      @provider = opts.fetch("provider")
+      @description = opts.fetch("description")
+      @documentation = opts.fetch("documentation")
     end
 
     def valid?