[BUGFIX beta] Allow a reduceComputed to have an undefined initialValue.

- Stop `ReduceComputedProperty.prototype.initialValue` from asserting
  that a reduce computed's initialValue must not be undefined.
- Modify `Ember.reduceComputed`'s assertion that initialValue be present
  to check that the initialValue is in options instead of isNone.
- Add a test that relies on an undefined initialValue to pass.
- Add a test that an error is thrown when you define a reduceComputed
  without an initialValue property.
[BUGFIX beta] Fix IE 6/7 RenderBuffer issue

IE 6/7 has a bug that prevents the 'name' attribute from being properly set on
an 'input' created with 'createElement'. This provides a workaround by using IE
specific `createElement` behavior that allows a tag string to be passed to
`createElement` instead of just a tagName.

Fixes #3525
Merge pull request #3518 from jamesotron/reduce-computed-undefined

Remove reduceComputed's requirement that initialValue not be undefined.
Merge pull request #3553 from wagenet/ie-render-buffer-fix

Fixes to RenderBuffer for IE6/7 - Fixes #3525