Fix creation of metadata with no SLS, when using settings.get_sp_metadata()

[bradenmacdonald]

Currently, if one wishes to generate SAML metadata using:

saml_settings = OneLogin_Saml2_Settings(config)
metadata = saml_settings.get_sp_metadata()

an error will occur if your settings do not specify a singleLogoutService URL in the config. This is meant to be optional, but the settings parser always adds an empty singleLogoutService dict to the settings dict, which gets interpreted by other parts of the code as meaning that singleLogoutService should be used. The fix checks for the presence of settings['singleLogoutService']['url'], which should only be present when the user actually wants to use a singleLogoutService.

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This is a contribution from edX, developed by OpenCraft as part of bringing integrated SAML support to the edX platform, via a new python-social-auth SAML backend.

[coveralls]

Coverage remained the same at 97.9% when pulling b1234fd on open-craft:edx1-sls-optional into c44971e on onelogin:master.