Fix newline stripping logic with WebTest 2.0.6

Background:

- Browsers strip the first newline and only the first newline from the
  contents of a <textarea> element.

- zope.testbrowser wants to mimic a real browser in this respect
  (https://bugs.launchpad.net/zope3/+bug/98371)

- WebTest 2.0.5 didn't strip the first newline, so zope.testbrowser had
  to do the stripping by itself.

- WebTest 2.0.6 started stripping the first newline
  (https://github.com/Pylons/webtest/pull/69)

Problem:

- this broke zope.testbrowser

Solution:

- require WebTest >= 2.0.6

- do not strip any newlines, expect WebTest to do that for us