Added possibility to set a custom `valueKey` for the parsed values from ...

...XML SOAP Response as the current fixed "$value" key could collide with some policies (as it's the case when using MongoDB).

**Background:**
As my team worked on another project which includes `MongoDB` as something like a `cache` for the data requested from the corresponding `wsdl` we ran into the issue that `$` is reserved and therefore not valid for keys to begin with, when working with `MongoDB`. So we had to write our own little function to rewrite every single `$value` key to something else.

So, I thought maybe it is a good idea to make it optional to work with a `valueKey` which could be defined in the `options`, just like the `attributesKey` can be set now.