Add support for nested as (#558)

This PR adds support for nested as :

```ruby
as :root do
  as :other_user do
    execute :cat, 'file', :strip => false
  end
end
```

Author: ThomasSevestre

lib/sshkit/backends/abstract.rb

@@ -103,6 +103,7 @@ def with(environment, &_block)
       end
 
       def as(who, &_block)
+        who_old = [@user, @group]
         if who.is_a? Hash
           @user  = who[:user]  || who["user"]
           @group = who[:group] || who["group"]
@@ -118,8 +119,7 @@ def as(who, &_block)
         EOTEST
         yield
       ensure
-        remove_instance_variable(:@user)
-        remove_instance_variable(:@group)
+        @user, @group = *who_old
       end
 
       class << self

test/unit/backends/test_abstract.rb

@@ -111,6 +111,61 @@ def test_within_home
         assert_equal 'cd ~/foo && /usr/bin/env cat file', backend.executed_command.to_command
       end
 
+      def test_as_properly_clears
+        backend = ExampleBackend.new do
+          as :root do
+            execute :cat, 'file', :strip => false
+          end
+
+          execute :cat, 'file', :strip => false
+        end
+
+        backend.run
+
+        assert_equal '/usr/bin/env cat file', backend.executed_command.to_command
+      end
+
+      def test_as_root
+        backend = ExampleBackend.new do
+          as :root do
+            execute :cat, 'file', :strip => false
+          end
+        end
+
+        backend.run
+
+        assert_equal 'sudo -u root -- sh -c /usr/bin/env\\ cat\\ file', backend.executed_command.to_command
+      end
+
+      def test_nested_as
+        backend = ExampleBackend.new do
+          as :root do
+            as :other_user do
+              execute :cat, 'file', :strip => false
+            end
+          end
+        end
+
+        backend.run
+
+        assert_equal 'sudo -u other_user -- sh -c /usr/bin/env\\ cat\\ file', backend.executed_command.to_command
+      end
+
+      def test_nested_as_properly_clears
+        backend = ExampleBackend.new do
+          as :root do
+            as :other_user do
+              execute :cat, 'file', :strip => false
+            end
+            execute :cat, 'file', :strip => false
+          end
+        end
+
+        backend.run
+
+        assert_equal 'sudo -u root -- sh -c /usr/bin/env\\ cat\\ file', backend.executed_command.to_command
+      end
+
       def test_background_logs_deprecation_warnings
         deprecation_out = +''
         SSHKit.config.deprecation_output = deprecation_out