Add support for WebDAV methods

[matthew16550]

I'm trying to test a WebDAV service with Tavern but cannot because schemas/tests.schema.yaml does not allow the extra WebDAV methods (from RFC 4918):

• COPY
• LOCK
• MKCOL
• MOVE
• PROPFIND
• PROPPATCH
• UNLOCK

I'm happy to make a PR adding them to the schema if you would like to support them?

[michaelboulton]

Feel free to make a PR - I've never used WebDAV, I assume it won't require any changes in functionality?