webdav: advertise PUT for existing files
webdav: advertise PUT for existing files

Change-Id: I285f93e562ca0c6bdc6168e6c47b28c7caef7cf5
Reviewed-on: https://go-review.googlesource.com/3501
Reviewed-by: Nick Cooper <d8c7b1721c483bd3ff883e3abbcd3cf7133480b3@google.com>
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

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