builder: fix escaping for ENV variables.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #8639 from erikh/fix_escapes

builder: fix escaping for ENV variables.
Add a testcase to make sure we don't squash tabs or convert them to spaces

This is in response to @SvenDowideit asking if we had a "tab" testcase
in https://github.com/docker/docker/issues/2315#issuecomment-58133508
I couldn't find one so I'm adding one

Closes #2315

Signed-off-by: Doug Davis <dug@us.ibm.com>
Merge pull request #8483 from duglin/TabTest

Add a testcase to make sure we don't squash tabs or convert them to spaces