Update textParser for Date objects

I ran in to trouble with Date parsing in this module because I have been working on Trireme (https://github.com/apigee/trireme) which is an alternate implementation of Node.js.

Trireme's JavaScript engine (Rhino) seems to interpret the default values when parsing Date strings differently than Node. I can try to track down the differences between Node and Rhino, and fix them in Rhino, but it's not clear to me at the moment that Node is necessarily "more correct."

Instead, I simplified the date parsing code and had it use methods that are less ambiguous about what they mean.

Can you take a look at this -- I think that it simplifies the code and does less harm so it's worth taking IMHO. Thank!
Instead of changing this test can you add another test?  That way both the case with the `Z` and the case where the `Z` is missing will be tested.  I try to avoid accepting a PR where a test change is required because the original test (for the missing timezone indicator) was added for a reason.