Replace carriage return with WebDriver "ENTER" constant

When the value is set in some input fields a carriage return was sent to
simulate pressing the enter key and thus confirming the input. However,
different browsers use different keys (Firefox uses "\r", but Chrome
uses "\n"), so the carriage return was replaced with the WebDriver
"ENTER" constant which is common to both browsers.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>