Fix mediadb str issues

[palfrey]

There's an issue with the use of str in mediadb, and a lack of tests for it meant it didn't turn up, so I've fixed that.

[codecov[bot]]

Codecov Report

Merging #44 (51db456) into master (25e6a85) will decrease coverage by 0.25%. The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   47.27%   47.02%   -0.25%     
==========================================
  Files          50       52       +2     
  Lines       10826    11408     +582     
==========================================
+ Hits         5118     5365     +247     
- Misses       5708     6043     +335     

Flag	Coverage Δ
unittest	47.02% <63.63%> (-0.25%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
coherence/backends/fs_storage.py	43.26% <0.00%> (ø)
coherence/upnp/core/service.py	66.44% <0.00%> (ø)
coherence/upnp/devices/media_server.py	33.33% <ø> (ø)
coherence/base.py	58.23% <40.00%> (-0.44%)	:arrow_down:
coherence/upnp/core/device.py	73.41% <66.66%> (-0.06%)	:arrow_down:
coherence/backends/mediadb_storage.py	54.22% <100.00%> (ø)
coherence/upnp/devices/control_point.py	38.33% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[palfrey]

@opacam Any thoughts? I've just hit these again

[opacam]

:rocket: Merged Thanks again @palfrey !!!! :smile: