making "Depth:infinity" the default for MERGE

Currently, you can specify a "Depth:infinity" header for MERGE.
Since you can get the same effect by saying that a MERGE is
always depth infinity, and that you can just use the version URL
of a version-controlled collection if you want a "Depth:0" merge,
I vote that we remove the Depth header as a needless complication
to the MERGE method.

Any objections?

Cheers,
Geoff