Increase test coverage for servlet specification compliance

[deki]

Currently there are two ways of mapping request events to servlet requests. The classic way is covered by tests using com.amazonaws.serverless.proxy.internal.testutils.AwsProxyRequestBuilder, the new way using Spring Cloud is tested in com.amazonaws.serverless.proxy.spring.AwsSpringHttpProcessingUtilsTests. The latter doesn't cover all cases that were already implemented in previous years, so we'd need to transfer the tests.

Ideally we'd leverage the official TCK https://github.com/jakartaee/servlet/tree/master/tck to some extend.