Fix JSON marshalling for TPM 2.0 digests

[orangecms]

This is what it looks like as of now: https://github.com/orangecms/utk-web/commit/8eb18d0a6262b1d40b17379ca0d62280556d09e9#diff-37dfadeb85117fdfb03addf286bc6002bfac2f93091f7595e1a5b8d39093c199R11

It appears that marshalling heterogeneous data structures in Go is a bit involved. I'm trying to see how it can be done at all. There is a corresponding TODO: https://github.com/9elements/tpmtool/commit/6e6199438513719dad29f5a0198f407663584dcc#diff-be86e8a168afe2b653da62f7d8f9d44c907b55446fb34edb49ff861c01662e7cR143