Make AuthnRequest Signature and SigAlg optional

[truongnmt]

Motivation

In my use case, AuthnRequest doesn't need to be signed.
Our IdP is Azure which said the following:
https://docs.microsoft.com/en-us/azure/active-directory/develop/single-sign-on-saml-protocol#signature

Signature
Don't include a Signature element in AuthnRequest elements, as Azure AD does not support signed authentication requests.

Besides, by making AuthnRequest optional, when adding service-provider to provider store, we don't need to specify a signing key.

Relate issues

#168 (comment)

Changes

• Added instance variable exclude_signature, defined by options[:exclude_signature]
• Existing applications which are not specifying this option will run as usual (sign AuthnRequest and add Signature, SigAlg params in AuthnRequest URL)
• Will not sign and add Signature, SigAlg params in AuthnRequest URL if specify exclude_signature = true

Usage:

Saml::Bindings::HTTPRedirect.create_url(authn_request, exclude_signature: true)

[coveralls]

Coverage increased (+8.0e-05%) to 99.848% when pulling b300377 on truongnmt:feat/make-authn-request-signature-optional into e2d3e12 on digidentity:master.

[truongnmt]

@benoist Can you please have a look? Thanks!