Using desired_capabilities affects future sessions #217

When desired_capabilities is passed to Open Browser, the default DesiredCapabilities object for the requested browser (which is a class attribute) is changed for the remainder of the test run. Unfortunately, the Selenium project's examples for Python demonstrate doing it that way.
I will submit a pull request to address this issue.
If instead we took a copy of the default desired capabilities object and manipulated that, this would not be an issue. A deep copy would safeguard against default capabilities with nested dicts.

In the following set of test cases, Second will use the proxy setup in First, even if they reside in different suites. You can see this in the output of Open Browser in Second if you run with log level DEBUG:
20130625 16:49:20.017 : DEBUG : POST http://127.0.0.1:4444/wd/hub/session {"sessionId": null, "desiredCapabilities": {"platform": "ANY", "browserName": "chrome", "version": "", "proxy": {"proxyType": "MANUAL", "httpProxy": "localhost:8888"}, "javascriptEnabled": true}}

First
    ${chrome dc}=    Evaluate    selenium.webdriver.DesiredCapabilities.CHROME    selenium
    Dictionary Should Not Contain Key    ${chrome dc}    proxy
    ${desired capabilities}=    Evaluate    {'proxy': {'proxyType': 'MANUAL', 'httpProxy': 'localhost:8888'}}
    Open Browser    http://www.google.com/    gc    remote_url=http://localhost:4444/wd/hub    desired_capabilities=${desired capabilities}
    [Teardown]    Close Browser

Second
    ${chrome dc}=    Evaluate    selenium.webdriver.DesiredCapabilities.CHROME    selenium
    Run Keyword And Continue On Failure    Dictionary Should Not Contain Key    ${chrome dc}    proxy
    Open Browser    http://www.msn.com/    gc    remote_url=http://localhost:4444/wd/hub
    [Teardown]    Close Browser