Improve errors method check in be_valid matcher (#2096)

If the `.be_valid?` matcher fails, it checks for the presence of a `.errors` method and uses it, if present, in the failure message.  This works great, unless the subject happens to include an `errors` method that takes an argument, in which case we get an error (because we call
errors with 0 arguments), this checks the arity of that method before we call it.

lib/rspec/rails/matchers/be_valid.rb

@@ -15,7 +15,7 @@ def matches?(actual)
         def failure_message
           message = "expected #{actual.inspect} to be valid"
 
-          if actual.respond_to?(:errors)
+          if actual.respond_to?(:errors) && actual.method(:errors).arity < 1
             errors = if actual.errors.respond_to?(:full_messages)
                        actual.errors.full_messages
                      else

spec/rspec/rails/matchers/be_valid_spec.rb

@@ -24,9 +24,19 @@ def valid?
     end
   end
 
+  class Car
+    def valid?
+      false
+    end
+
+    def errors(_)
+    end
+  end
+
   let(:post) { Post.new }
   let(:book) { Book.new }
   let(:boat) { Boat.new }
+  let(:car) { Car.new }
 
   it "includes the error messages in the failure message" do
     expect {
@@ -46,6 +56,12 @@ def valid?
     }.to raise_exception(/expected .+ to be valid\z/)
   end
 
+  it "includes a brief error message when error message is wrong arity" do
+    expect {
+      expect(car).to be_valid
+    }.to raise_exception(/expected .+ to be valid\z/)
+  end
+
   it "includes a failure message for the negative case" do
     allow(post).to receive(:valid?) { true }
     expect {