webdav: have escapeXML perform fewer allocations.

escapeXML was introduced in the previous commit:
https://go-review.googlesource.com/29297

Change-Id: I7d0c982049e495b312b1b8d28ba794444dd605d4
Reviewed-on: https://go-review.googlesource.com/32370
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed