Fix tempfile returning nil for empty files

This is an "monkey patch" to have Tempfile return 0 instead of nil when
a tempfile is empty. This has caused an unexplained exception in our
logs for years, and I was really surprised when I caught it, and more
surprised that nobody has ever reported it to us before.

I really think this is a bug, and have filed it as a bug on the ruby
tracker. I haven't seen any consequences (yet) to doing this, so I'm
just going to run with it and see what happens. If it blows
something up (how? what scenario?), I'll try a different approach.

ext/tempfile.rb

@@ -0,0 +1,7 @@
+class Tempfile
+  alias_method :size_original, :size
+  def size
+    s = size_original
+    s.nil? ? 0 : s
+  end
+end

tests/tempfile_tests.rb

@@ -0,0 +1,10 @@
+require_relative './environment.rb'
+
+describe Tempfile do
+  it 'should return 0 when no data is written' do
+    tmp = Tempfile.new
+    tmp.write ''
+    tmp.close
+    tmp.size.must_equal 0
+  end
+end