allow setSpEntityId
Merge pull request #89 from miremond/pac4j-18

allow setSpEntityId
Two fixes related to signing. First, we were incorrectly setting SP AuthnRequestSigned always to true even when not signing requests. Secondly, when an IdP says WantAuthnRequestsSigned it simply means that it supports signed requests, but not that requests must be signed, so this fixes it so that those requests complete successfully instead of throwing a TechnicalException