Ensure Arbre context is reset

When you have an Arbre component with a block form, it pushes the current
node onto a stack so that it can use [method_missing][1] to provide a dynamic
evaluation context (eg. so tab is available only directly inside of tabs).

Unfortunately, when an error is raised inside inside one of these, if we
have a rescue to catch that error so the rest of the page can be rendered -
the context is still stuck inside the last block that was added. Other than
likely rendering some of the wrong markup (depending on usage), it means
the context is not put back into the block where the rescue is located.
As a result, even though that block is effectively thrown away when a
rescue is located above the erroring call, certain features are not available.

[1]: https://github.com/activeadmin/arbre/blob/v2.2.0/lib/arbre/element.rb#L176-L186

Author: drcapulet

lib/arbre/context.rb

@@ -90,6 +90,7 @@ def with_current_arbre_element(tag)
       raise ArgumentError, "Can't be in the context of nil. #{@_current_arbre_element_buffer.inspect}" unless tag
       @_current_arbre_element_buffer.push tag
       yield
+    ensure
       @_current_arbre_element_buffer.pop
     end
     alias_method :within, :with_current_arbre_element

spec/arbre/unit/context_spec.rb

@@ -32,4 +32,30 @@
     expect(context.index('<')).to eq(0)
   end
 
+  context "when an error is raised in a nested block" do
+    let(:context) do
+      described_class.new do
+        div do
+          para 'hello!'
+          div do
+            ul do
+              li do
+                'item one'
+              end
+              li do
+                raise 'error talking to the db'
+              end
+            end
+          rescue StandardError
+            para 'Uh oh! DB call failed'
+          end
+        end
+      end
+    end
+
+    it "properly resets the element buffer" do
+      # When this example fails, it causes a SystemStackError
+      expect(context.current_arbre_element).to be_a(described_class)
+    end
+  end
 end