SAML2AuthenticationHandler#handleRequestType should check request method against HttpContext Request instance instead of AuthnRequestType

The fix of PLINK-700 introduces a regression in HTTP Redirect workflow use-cases (between SP and IdP) by the following change:

-                boolean isPost = httpContext.getRequest().getMethod().equalsIgnoreCase("POST");
+               boolean isPost = art.getProtocolBinding().toString().equals(JBossSAMLURIConstants.SAML_HTTP_POST_BINDING.get());

Even if httpContext.getRequest() has set GET http method, the handleRequestType method now assumes it being POST in case of urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST in AuthnRequestType.