Fix Data object support

[mloar]

Restler supplies a Data object, but it is improperly handled when writing to the stream.

1. Omission of an 'else' results in the metadata associated with the object not being written in the part header.
2. Omission of handling when writing the value results in "[Object object]" being written to the stream.

This corrects these two issues.

[lapo-luchini]

I hope this gets in an npm release soon, as depending on a github fork (as opposed to a package) is more difficult to npm install on some production servers that can only access http.

[easternbloc]

@lapo-luchini can you write a test for this? I'm happy to merge it once it has coverage...

[lapo-luchini]

OK, I will try and do that.

[lapo-luchini]

I added a test case for this in lapo-luchini/restler@5b2d4cc3b500d7c305dfcce5bdf615154a67dcee, but I can't produce a clean pull request from that branch as I imported both changes from #119 and #108.
Can you pluck the test, or should I create a brand new branch to pull from?

[easternbloc]

If you can make a fresh PR that would be greatly appreciated. :)

[lapo-luchini]

OK, everything's in the new PR #172; I hope the testcase is ok, it's the first I write with nodeunit, but duplicating an existing example was easy enough.
(also: notice that there are no tests for multi-part using File content, so this test for Data is adding actual coverage also to the common parts)