Fix Docker::Image.create

The credential-header-logic was a bit off, this commit fixes it and adds
a test.

lib/docker/image.rb

@@ -107,13 +107,10 @@ class << self
 
     # Create a new Image.
     def create(opts = {}, creds = nil, conn = Docker.connection)
-      credentials = (creds.nil?) ? creds.to_json : Docker.creds
-      headers = if credentials.nil?
-        Docker::Util.build_auth_header(credentials)
-      else
-        {}
-      end
-      body = conn.post('/images/create', opts)
+      credentials = creds.nil? ? Docker.creds : creds.to_json
+      headers = !credentials.nil? && Docker::Util.build_auth_header(credentials)
+      headers ||= {}
+      body = conn.post('/images/create', opts, :headers => headers)
       id = Docker::Util.fix_json(body).last['id']
       new(conn, 'id' => id, :headers => headers)
     end

spec/docker/image_spec.rb

@@ -214,14 +214,24 @@
     subject { described_class }
 
     context 'when the Image does not yet exist and the body is a Hash' do
-      let(:image) { subject.create('fromImage' => 'base') }
+      let(:image) { subject.create('fromImage' => 'ubuntu') }
+      let(:creds) {
+        {
+          :username => 'nahiluhmot',
+          :password => '*********',
+          :email => '[email protected]'
+        }
+      }
+
+      before { Docker.creds = creds }
 
-      it 'sets the id', :vcr do
+      it 'sets the id and sends Docker.creds', :vcr do
         image.should be_a Docker::Image
         image.id.should match(/\A[a-fA-F0-9]+\Z/)
         image.id.should_not include('base')
         image.id.should_not be_nil
         image.id.should_not be_empty
+        image.info[:headers].keys.should include('X-Registry-Auth')
       end
     end
   end