Don't listen for SIGPIPE

SIGPIPE normally doesn't cause node to exit [1]. However, by registering
and unregistering a "SIGPIPE" event handler, the behavior changes and
node starts exiting when receiving SIGPIPE signals.

Because signal-exit listens for SIGPIPE, it causes programs to exit
when receiving SIGPIPE, whereas normally node programs not using
signal-exit should not exit when receiving SIGPIPE.

This changes signal-exit to not listen for SIGPIPE and tests that a
program using signal-exit will not exit due to SIGPIPE signals. The test
uses an existing test fixture (test/fixtures/sigpipe.js) that was not
previously being used.

See also #19.

[1] https://github.com/nodejs/node/blame/44aba1ab1c2656caf8f0f03a2f47b4c4b35aa4a2/doc/api/process.markdown#L243

signals.js

@@ -23,7 +23,6 @@ module.exports = [
   'SIGILL',
   'SIGINT',
   'SIGIOT',
-  'SIGPIPE',
   'SIGPROF',
   'SIGQUIT',
   'SIGSEGV',

test/signal-exit-test.js

@@ -102,4 +102,13 @@ describe('signal-exit', function () {
       done()
     })
   })
+
+  it('does not exit on sigpipe', function (done) {
+    exec(process.execPath + ' ./test/fixtures/sigpipe.js', shell, function (err, stdout, stderr) {
+      assert.ifError(err)
+      stdout.should.match(/hello/)
+      stderr.should.match(/onSignalExit\(0,null\)/)
+      done()
+    })
+  })
 })