fix packing of date objects in hashmaps

[dstokes]

packing of objects with new Date() objects as values results in the value being converted to {}. this change converts date objects to timestamp strings

[dogada]

Drew, thanks for your patch, but decoder should restore exactly same objects that encoder stored. We can't store Date and restore String.

There is no Date data type in JSON format and people usually converts Dates to a string representation before calling JSON.stingify or use a JSON extension like EJSON for automatic Date conversion.

You can also look for other popular approaches on SO: http://stackoverflow.com/questions/10286204/the-right-json-date-format

[dstokes]

maybe something added to the docs then? i can't be the only person who's run into this gotcha ;)

[dogada]

Good idea. Updated docs and added some tests in 8d8e4c7