within_frame should allow no arguments when theres only one frame in scope

lib/capybara/session.rb

@@ -397,6 +397,8 @@ module Capybara
           scopes.slice!(idx..-1)
           driver.switch_to_frame(:top)
         end
+      else
+        raise ArgumentError, "You must provide a frame element, :parent, or :top when calling switch_to_frame"
       end
     end
 
@@ -846,6 +848,8 @@ module Capybara
     end
 
     def _find_frame(*args)
+      return find(:frame) if args.length.zero?
+
       case args[0]
       when Capybara::Node::Element
         args[0]

lib/capybara/spec/session/frame/within_frame_spec.rb

@@ -54,6 +54,20 @@ Capybara::SpecHelper.spec '#within_frame', requires: [:frames] do
     end
   end
 
+  it "should default to the :frame selector when no options passed" do
+    container = @session.find(:css, '#divInMainWindow')
+    @session.within(container) do
+      # Ensure only one frame in scope
+      @session.within_frame do
+        expect(@session).to have_css('body#parentBody')
+      end
+    end
+    expect {
+      # Multiple frames in scope here
+      @session.within_frame do; end
+    }.to raise_error Capybara::Ambiguous
+  end
+
   it "should find multiple nested frames" do
     @session.within_frame 'parentFrame' do
       @session.within_frame 'childFrame' do