add sslProxy to Client.add_to_capabilities

when i use following code

    from browsermobproxy import Client
    from selenium import webdriver
    proxy = Client('127.0.0.1:9090')
    capabilities  = webdriver.DesiredCapabilities.FIREFOX
    proxy.add_to_capabilities(capabilities)

    driver = webdriver.Remote(
         command_executor='http://127.0.0.1:4444/wd/hub', desired_capabilities=capabilities
    )
    proxy.new_har('testHTTPSHar')

all HTTP traffic saved to HAR file
but HTTPS traffic missed

i added small improvment in add_to_capabilities method