patch 8.1.0269: Ruby Kernel.#p method always returns nil

Problem:    Ruby Kernel.#p method always returns nil.
Solution:   Copy p method implementation from Ruby code. (Masataka Pocke
            Kuwabara, closes #3315)
diff --git a/src/testdir/test_ruby.vim b/src/testdir/test_ruby.vim
index 8d7943e..5476e51 100644
--- a/src/testdir/test_ruby.vim
+++ b/src/testdir/test_ruby.vim
@@ -363,4 +363,17 @@
   ruby p 'Just a test'
   let messages = split(execute('message'), "\n")
   call assert_equal('"Just a test"', messages[-1])
+
+  " Check return values of p method
+
+  call assert_equal('123', RubyEval('p(123)'))
+  call assert_equal('[1, 2, 3]', RubyEval('p(1, 2, 3)'))
+
+  " Avoid the "message maintainer" line.
+  let $LANG = ''
+  messages clear
+  call assert_equal('true', RubyEval('p() == nil'))
+
+  let messages = split(execute('message'), "\n")
+  call assert_equal(0, len(messages))
 endfunc