Ensure NmtBase.state always returns a string

[erlend-aasland]

Fixes #500

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 68.75%. Comparing base (fa74236) to head (b871cf2).

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Additional details and impacted files

[](https://app.codecov.io/gh/christiansandberg/canopen/pull/506?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Christian+Sandberg) ```diff @@ Coverage Diff @@ ## master #506 +/- ## ========================================== + Coverage 68.74% 68.75% +0.01% ========================================== Files 26 26 Lines 3116 3117 +1 Branches 527 526 -1 ========================================== + Hits 2142 2143 +1 Misses 837 837 Partials 137 137 ``` | [Files](https://app.codecov.io/gh/christiansandberg/canopen/pull/506?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Christian+Sandberg) | Coverage Δ | | |---|---|---| | [canopen/nmt.py](https://app.codecov.io/gh/christiansandberg/canopen/pull/506?src=pr&el=tree&filepath=canopen%2Fnmt.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Christian+Sandberg#diff-Y2Fub3Blbi9ubXQucHk=) | `93.23% <100.00%> (+0.05%)` | :arrow_up: |