[release-branch.r60] gofix: do not convert url in field names

[release-branch.r60] gofix: do not convert url in field names

««« CL 4972052 / 0f7a647510f9
gofix: do not convert url in field names

There's some ambiguity in the U{url: url} case as it could be
both a map or a struct literal, but given context it's more
likely a struct, so U{url: url_} rather than U{url_: url_}.
At least that was the case for me.

R=golang-dev, rsc, adg
CC=golang-dev
https://golang.org/cl/4972052
»»»

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4962058