Take raiseHttpErrors into account on submit

If an HTTPError is raised on `browser.submit`, it should only be
reraised to the user if `browser.raiseHttpErrors=True`.

This behavior already applies to `browser.open`, thus it is expected to
apply to `browser.submit` as well.
Add tests confirming redirect behavior

The following functional tests are added:

* Perform redirect on browser.submit if 30x is returned

If `browser.mech_browser.set_handle_redirects(False)` is called and
`browser.raiseHttpErrors` is set to `False`, redirects shouldn't be
performed automatically:

* Do not perform redirect on browser.open if 30x is returned
* Do not perform redirect on browser.submit if 30x is returned

The last of the 3 added tests tests the code introduced in GitHub
zopefoundation/zope.testbrowser#4, while the first two tests confirm
prior behavior.
Merge pull request #4 from nidico/use-raiseHttpErrors-on-submit

Take raiseHttpErrors into account on submit