webdav: log a non-nil error for unsupported HTTP methods.
webdav: log a non-nil error for unsupported HTTP methods.

Change-Id: Idcd7c225efda94021ba18ca33f3fe7e8565140fe
Reviewed-on: https://go-review.googlesource.com/3861
Reviewed-by: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>
Reviewed-by: Nick Cooper <d8c7b1721c483bd3ff883e3abbcd3cf7133480b3@google.com>

zabawaba99/net,raboof/net,veliamsli/net,kiip/net,zabawaba99/net,mckinney-subgraph/net,albertjin/golang-net,chenzlabs/net,DramaFever/golang.org-x-net,cmars/tools,albertjin/golang-net,cesanta/goxnet,foreversmart/net,subuk/net,cesanta/goxnet,gomini/net,yangxianjun/net,gomini/net,cmars/tools,kiip/net,w4ngyi/net,dafyddcrosby/net,w4ngyi/net,cmars/tools,subuk/net,veliamsli/net,yangxianjun/net,flowlo/go-net,dafyddcrosby/net,flowlo/go-net,muzining/net,golang/net,foreversmart/net,mckinney-subgraph/net,muzining/net,raboof/net,DramaFever/golang.org-x-net,cmars/tools,chenzlabs/net