Checking for null to prevent a NPE when no Origin is specified in JSR356Endpoint

When solving an issue with Undertow having case-sensitive headers (https://issues.jboss.org/browse/UNDERTOW-252) a second check for `null` at `org.atmosphere.container.JSR356Endpoint` line 161 should be made. When no Origin header is specified a NullPointerException is thrown.

P.S.: Sorry about the imports, dunno why eclipse liked them better that way