html: propagate foreign namespaces only when adding foreign content.

Pass tests10.dat, test 31:
<div><svg><path><foreignObject><p></div>a

| <html>
|   <head>
|   <body>
|     <div>
|       <svg svg>
|         <svg path>
|           <svg foreignObject>
|             <p>
|               "a"

Also pass test 32:
<!DOCTYPE html><svg><desc><div><svg><ul>a

R=andybalholm
CC=golang-dev
https://golang.org/cl/5527064
2 files changed