Add invalid params error handling for Saml::Bindings::HTTPPost.receive_message

[soartec-lab]

Thanks for creating a great gem.
I created this PR because I found one problem while developing a project using this gem. Please check if you like.

Summary

If both SAMLRequest and SAMLResponse are nil in the request parameter, the argument nil is specified in Saml::Encoding#decode_64 and the exception NoMethodError is raised.

$ Saml::Encoding.decode_64(nil)
NoMethodError: undefined method `unpack1' for nil:NilClass
from /usr/local/lib/ruby/2.7.0/base64.rb:59:in `decode64'

See: https://github.com/digidentity/libsaml/blob/master/lib/saml/bindings/http_post.rb#L23

This isn't friendly to developers, so I've added an exception class Saml::Errors::InvalidParams to detect this.

[coveralls]

Coverage remained the same at 99.848% when pulling 428e397 on soartec-lab:feature/add-invalid-params-error into a09fe2f on digidentity:master.

[jdongelmans]

Thanks, version v3.9.1 has just been pushed!