Match Hash implementation of fetch

An optional block can be passed to `fetch` that will be called in the
event that the variable is not set.

    fetch(:var) { fail 'var not found' }

lib/capistrano/dsl/env.rb

@@ -6,8 +6,8 @@ module Capistrano
         env.configure_backend
       end
 
-      def fetch(key, default=nil)
-        env.fetch(key, default)
+      def fetch(key, default=nil, &block)
+        env.fetch(key, default, &block)
       end
 
       def any?(key)

spec/integration/dsl_spec.rb

@@ -165,6 +165,20 @@ describe Capistrano::DSL do
       end
     end
 
+    context 'with a block' do
+      context 'when the variables is defined' do
+        it 'returns the variable' do
+          expect(dsl.fetch(:scm) { :svn }).to eq :git
+        end
+      end
+
+      context 'when the variables is undefined' do
+        it 'calls the block' do
+          expect(dsl.fetch(:source_control) { :svn }).to eq :svn
+        end
+      end
+    end
+
   end
 
   describe 'asking for a variable' do