Remove support for passing nil to IO#ungetc

Fixes [Bug #13675]

io.c

@@ -4350,7 +4350,6 @@ rb_io_ungetc(VALUE io, VALUE c)
 
     GetOpenFile(io, fptr);
     rb_io_check_char_readable(fptr);
-    if (NIL_P(c)) return Qnil;
     if (FIXNUM_P(c)) {
 	c = rb_enc_uint_chr(FIX2UINT(c), io_read_encoding(fptr));
     }

spec/ruby/core/io/ungetc_spec.rb

@@ -103,10 +103,19 @@ describe "IO#ungetc" do
     -> { @io.sysread(1) }.should raise_error(IOError)
   end
 
-  it "does not affect the stream and returns nil when passed nil" do
-    @io.getc.should == ?V
-    @io.ungetc(nil)
-    @io.getc.should == ?o
+  ruby_version_is "0"..."2.8" do
+    it "does not affect the stream and returns nil when passed nil" do
+      @io.getc.should == ?V
+      @io.ungetc(nil)
+      @io.getc.should == ?o
+    end
+  end
+
+  ruby_version_is "2.8" do
+    it "raises TypeError if passed nil" do
+      @io.getc.should == ?V
+      proc{@io.ungetc(nil)}.should raise_error(TypeError)
+    end
   end
 
   it "puts one or more characters back in the stream" do