cURL: Support custom HTTP methods

[ocean90]

See https://core.trac.wordpress.org/ticket/37503 for background.

The test doesn't work (yet) because there is no /purge endpoint.

[codecov-io]

Current coverage is 92.22% (diff: 100%)

Merging #227 into master will increase coverage by <.01%

@@             master       #227   diff @@
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  Files            21         21          
  Lines          1761       1762     +1   
  Methods         156        156          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits           1624       1625     +1   
  Misses          137        137          
  Partials          0          0          

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[rmccue]

https://github.com/RequestsPHP/test-server is the source for the test server. :)

[ocean90]

@rmccue No idea why it returns a 501 error. :(

[ocean90]

Instead of adding a new case we can also combine the default case with PATCH, PUT, DELETE and OPTIONS. See https://core.trac.wordpress.org/attachment/ticket/37503/37503.2.patch.

@rmccue Let me know if this is a better approach, happy to refresh the PR. (It would also fix the coverage thing. :))

[rmccue]

Coverage can be pretty safely ignored for <0.1%; I tried this PR locally and it worked, but not on Travis, so need to get to the bottom of that. Haven't had time yet, alas.

I'd prefer the switch be explicit in what it supports, but case ...: default: is fine to avoid the duplication and still be explicit.

[rmccue]

Tests passing, wunderbar!