json/yaml/xml writer add redundant "Document" property at top level

[nicoweidner]

The writer for json/yaml/xml writes documents like the following, wrapping all content in a redundant Document property at top level:

{
    "Document": {
        "spdxVersion": "SPDX-2.1",
        "documentNamespace": "https://spdx.org/spdxdocs/spdx-example-444504E0-4F89-41D3-9A0C-0305E82C3301",
        <...>

Apart from being redundant, this is incompatible with e.g. the java spdx library, and it is also incompatible with the json spec.

Note: I feel I have seen this issue before, but I couldn't find it now.

[meretp]

relates to #184

[nicoweidner]

Comment: In the case of xml, there needs to be a root element, see https://www.w3schools.com/xml/xml_syntax.asp. But for json and yaml, this is not required.

[meretp]

fixed with #254