Add specs for Kernel#p return values

Kernel#p's return value had no coverage for 3 scenarios:

- nil for no arguments
- the argument itself (not a 1-element Array) for a single argument
- a new Array for multiple arguments

That passthrough is what lets `p` be used inline where `puts` cannot, since `puts` always returns nil.

The nil cases are asserted on the existing "prints nothing" examples, as `core/io/puts_spec.rb` does for IO#puts. The passthrough cases get their own examples, as `core/io/output_spec.rb` does for IO#<<. Specs are under Kernel#p only, following a1f57e8.

Matchers follow #1350 (.should == nil, .should.equal?).

Author: edgibbs

core/kernel/p_spec.rb

@@ -69,11 +69,32 @@
   end
 
   it "prints nothing if no argument is given" do
-    -> { p }.should output("")
+    -> { p.should == nil }.should output("")
   end
 
   it "prints nothing if called splatting an empty Array" do
-    -> { p(*[]) }.should output("")
+    -> { p(*[]).should == nil }.should output("")
+  end
+
+  it "returns the argument if a single argument is given" do
+    o = mock("Inspector Gadget")
+    o.should_receive(:inspect).any_number_of_times.and_return "Next time, Gadget, NEXT TIME!"
+
+    -> { p(o).should.equal?(o) }.should output("Next time, Gadget, NEXT TIME!\n")
+  end
+
+  it "returns the argument if called splatting a single-element Array" do
+    o = mock("Inspector Gadget")
+    o.should_receive(:inspect).any_number_of_times.and_return "Next time, Gadget, NEXT TIME!"
+
+    -> { p(*[o]).should.equal?(o) }.should output("Next time, Gadget, NEXT TIME!\n")
+  end
+
+  it "returns an Array of the arguments if multiple arguments are given" do
+    o = mock("Inspector Gadget")
+    o.should_receive(:inspect).any_number_of_times.and_return "Next time, Gadget, NEXT TIME!"
+
+    -> { p(o, o).should == [o, o] }.should output("Next time, Gadget, NEXT TIME!\nNext time, Gadget, NEXT TIME!\n")
   end
 
   # Not sure how to spec this, but wanted to note the behavior here