Finishing Coverage for /rmpv/src/decode/value.rs

[sacherjj]

I was reading through code base as a Rust learning exercise and added some more test coverage.

This gives at least normal case coverage for everything in /rmpv/src/decode/value.rs, with the exception of Marker::Reserved.

[sacherjj]

Note: I did a reset back to 537fa0d and new commit to clean up my commits before Pull request. So it looks like that is the 2nd commit that is part of this.

[coveralls]

Coverage increased (+0.9%) to 87.099% when pulling 537fa0d on sacherjj:master into f4184f2 on 3Hren:master.

[coveralls]

Coverage increased (+0.9%) to 87.099% when pulling 537fa0d on sacherjj:master into f4184f2 on 3Hren:master.

[3Hren]

Thanks!