*** THIS IS A BREAKING CHANGE *** as per [1]: "Required is forever"

Nevertheless this seems like a good idea at this time: there are no known
producers ( nor consumers ) of MetaData nodes, and the current requirement
of MimeType has an extremely narrow application scope.

This change could very well be rejected in lieu of implementing a new type
of node ( e.g. TheRealMetadata ) in the DataType enum.

Based on https://github.com/ipfs/go-ipfs/issues/3451#issuecomment-264246718

License: MIT
Signed-off-by: Mib Kd743naq <mib.kd743naq@gmail.com>

[1] https://developers.google.com/protocol-buffers/docs/proto#specifying-field-rules