Fix creating sync subscription for the same path and different method…

[mthenw]

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: NO
Is it a breaking change?: NO

[codecov]

Codecov Report

Merging #468 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #468   +/-   ##
======================================
  Coverage    68.5%   68.5%           
======================================
  Files          34      34           
  Lines        2137    2137           
======================================
  Hits         1464    1464           
  Misses        612     612           
  Partials       61      61

Impacted Files	Coverage Δ
libkv/subscription.go	88.6% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2d9da41...d1b5df7. Read the comment docs.