exp/template/html: do not escape the RHS of assignments

exp/template/html: do not escape the RHS of assignments

In

  {{$x := . | foo}}
  {{$x}}

the first action is a variable assignment that contributes
nothing to the output while the first is a use that needs
to be escaped.

This CL fixes escapeAction to distinguish assignments from
interpolations and to only modify interpolations.

R=nigeltao, r
CC=golang-dev
https://golang.org/cl/5143048