Rename `setServerParameters()` to `haveServerParameters()`; drop `haveServerParameter()`

[ThomasLandauer]

Just stumbled upon this while searching for a new name for deleteHeader(), see #22

Current situation:

• haveServerParameter('name', 'value') uses Symfony's Client::setServerParameter()
• setServerParameters([]) uses Symfony's Client::setServerParameters() which sets 'HTTP_USER_AGENT' => 'Symfony BrowserKit' by default.

I would say:

• "setServerParameters" isn't the right term, Codeception's convention for this is "haveServerParameters".
• It doesn't make sense to have two functions for this, one for a single parameter, and one for multiple. If the expected argument is an array, anybody knows immediately what to pass. So the existing haveServerParameter can be deprecated in favor of the new haveServerParameters.
• Remaining question: Is the "Symfony BrowserKit" thing more of a bug or more of a feature?
  If feature: Just pass the array along to Symfony's setServerParameters, and that's it.
  If bug: foreach() over the array and use Symfony's setServerParameter.

[ThomasLandauer]

Both functions also exist in lib-innerbrowser:

• https://github.com/Codeception/lib-innerbrowser/blob/master/src/Codeception/Lib/InnerBrowser.php#L2061
• https://github.com/Codeception/lib-innerbrowser/blob/master/src/Codeception/Lib/InnerBrowser.php#L2047