Always save amount actually printed on the document

The `saveDocument` method used a different way to decide between netto and brutto than `assignValues4x`. Because of this, the amount shown in the document tab is incorrect when creating a document for a tax free order (`taxfree` flag in `s_order` is `1`) but unchecking the tax free checkbox in the backend. The document is still generated _without_ tax, but the amount displayed in the documents tab in the backend _does_ include tax.

We already documented this behavior with #178.

Instead of just merging this pull request, it would probably make more sense to also change the behavior of the `netto` flag for `Shopware_Components_Document::initDocument()` like this:
* If not set: Fall back to the `taxfree` flag from `s_order` as default
* If set to `true`: Generate a tax-free document
* If set to `false`: Generate the document with tax (this last part would theoretically be a backwards incompatible change and should be documented in `UPGRADE.md`)