Attempt to fix test-bundler

f7cf641469161c3770b58f79e08e312512212aa8 broke spec/bundler/install/gems/resolving_spec.rb:356.
This line seems to impact that test, so I slightly modified the
implementation for that spec's case.

lib/bundler/endpoint_specification.rb

@@ -12,7 +12,7 @@ module Bundler
       super()
       @name         = name
       @version      = Gem::Version.create version
-      @platform     = Gem::Platform.new(platform)
+      @platform     = platform.nil? ? nil : Gem::Platform.new(platform)
       @spec_fetcher = spec_fetcher
       @dependencies = dependencies.map {|dep, reqs| build_dependency(dep, reqs) }