Fix method assertion ensuring it will match "end" with the same indentation

Otherwise it was matching "end" lines from blocks within the method body,
making tests such as the migration ones fail.

railties/lib/rails/generators/test_case.rb

@@ -163,8 +163,8 @@ module Rails
       #     end
       #   end
       def assert_instance_method(method, content)
-        assert content =~ /def #{method}(\(.+\))?(.*?)\n\s+end/m, "Expected to have method #{method}"
-        yield $2.strip if block_given?
+        assert content =~ /(\s+)def #{method}(\(.+\))?(.*?)\n\1end/m, "Expected to have method #{method}"
+        yield $3.strip if block_given?
       end
       alias :assert_method :assert_instance_method