propfind: Allow user to specify depth, default to depth of 1.

[inferiorhumanorgans]

This'll allow users to specify an arbitrary propfind depth, as well as make it easier for users to override other headers in the future. Possible modifications include:

• validating the depth value (int or string of 'infinity')
• have the options merely insert values into the headers hash by default

[devrandom]

The patch loses the feature of specifying :acl for the body if the option hash is specified.

Perhaps make the body argument explicit and separate out the handling of :acl into its own if statement?