Collection Objects for Xml Serialization

[beberlei]

This expands on GH-117.

I introduced a collection object into my code. The following approaches failed:

1. ArrayCollection from Doctrine still lead to
2. A simple Traversable implementation lead to

It seems traversables get special handling in the GraphNavigator#104. I think this should not be the case if the metadata of the traversable is not null.

Additionally to solve the "" problem in xml serialization, there should somehow be a way to at least replace with something, i think the XmlSerializervisitor could use the root element name as entryName of each value somehow in visitArray(). That would even allow serialization of inheritances:

<result><object1/><object2/><object1/><object3/></result>

Ping @lsmith77

[schmittjoh]

I agree on not treating Traversables specially. Did you already work on a patch for this?

Regarding the entry name, I'm not sure, it could add some amount of performance penalty since we would have to determine it for each object separately, and then also support the generation in both directions. It gets even more complicated if someone uses the "inline" option in which case we do not even know to which property the entry would belong.

[mvrhov]

As I need something like @lsmith pointed out in #48 I've taken a crack at it what seemed to be possible.
Well everything was starting to come together, I implemented a VirtualArrayCollection with a specific interface which would then return a virtual class name, well the problem became when trying to load a metadata for that virtual class. Metadata factory requires a concrete class implementation to be able to load a metadata for.

[schmittjoh]

This should now be possible in the same fashion as #117 (see my comments there).