Bug 49739: Use named parameter if value contains '='

Fixes this test case:
echo '{{echo|foo}}' | node parse | sed s/foo/f=oo/g | node parse --html2wt
{{echo|1=f=oo}}

Added a test too.

Change-Id: I1619669f7112f8d1738abfe690656b1541e5b107
Merge "Bug 49739: Use named parameter if value contains '='"