bugfix - pSlice used, but not defined in eql.js

fix missing reference to pSlice in eql.js & include a unit test to verify...

unit test prior to eql.js change
```
  ✖ 1 of 52 tests failed:

  1)  test eql(val):
     ReferenceError: pSlice is not defined
      at objEquiv (/home/boden/git/should.js/lib/eql.js:61:9)
      at _deepEqual (/home/boden/git/should.js/lib/eql.js:38:12)
      at Object.Assertion.eql (/home/boden/git/should.js/lib/should.js:286:9)
      at Context.module.exports.test eql(val) (/home/boden/git/should.js/test/should.test.js:255:20)
      at Test.Runnable.run (/home/boden/git/should.js/node_modules/mocha/lib/runnable.js:213:32)
      at Runner.runTest (/home/boden/git/should.js/node_modules/mocha/lib/runner.js:343:10)
      at Runner.runTests.next (/home/boden/git/should.js/node_modules/mocha/lib/runner.js:389:12)
      at next (/home/boden/git/should.js/node_modules/mocha/lib/runner.js:269:14)
      at Runner.hooks (/home/boden/git/should.js/node_modules/mocha/lib/runner.js:278:7)
      at next (/home/boden/git/should.js/node_modules/mocha/lib/runner.js:226:23)
      at Runner.hook (/home/boden/git/should.js/node_modules/mocha/lib/runner.js:246:5)
      at process.startup.processNextTick.process._tickCallback (node.js:244:9)


make: *** [test] Error 1
```

after change:

```
52 tests complete (16 ms)
```