Check for nil instead of truthiness when diffing

When trying to make a diff where either the expected
or actual value was false, the diff would be an empty
string. This change makes it to diff against the false
value.

Author: Lucas Nestor

lib/rspec/support/differ.rb

@@ -11,7 +11,7 @@ class Differ
       def diff(actual, expected)
         diff = ""
 
-        if actual && expected
+        unless actual.nil? || expected.nil?
           if all_strings?(actual, expected)
             if any_multiline_strings?(actual, expected)
               diff = diff_as_string(coerce_to_string(actual), coerce_to_string(expected))

spec/rspec/support/differ_spec.rb

@@ -468,6 +468,13 @@ def inspect; "<BrokenObject>"; end
             expect(diff).to be_diffed_as(expected_diff)
           end
         end
+
+        context 'when expected or actual is false' do
+          it 'generates a diff' do
+            expect(differ.diff(true, false)).to_not be_empty
+            expect(differ.diff(false, true)).to_not be_empty
+          end
+        end
       end
     end
   end