Consistently accept empty Content-Type header and empty character encoding

Issue: SPR-12173
Polishing
EclipseLinkJpaDialect passes custom isolation level on to EclipseLink's DatabaseLogin configuration

Issue: SPR-12319
Improve HttpSessionHandshakeInterceptor

Use explicit flag whether to copy all attributes.
Check for duplicate keys in YAML map nodes

Snake YAML allows for duplicate keys in map nodes. See
https://code.google.com/p/snakeyaml/issues/detail?id=199

This commit uses a dedicated Constructor extension that explicitly
checks for such duplicate keys.

Issue: SPR-12318
Polish resource handling tests
Prepend leading slash in ResourceUrlProvider

The getForRequestUrl method of ResourceUrlProvider uses the
HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE attribute to
determine the relevant portion of the resource URL path.

However there are cases when that attribute may not have a leading
(e.g. when the current URL was matched to a prefix-based pattern
and hence extracted via PathMatcher#extractPathWithinPattern), which
interferes with the matching of resource URL paths to patterns.

This change ensures a leading slash is present

Issue: SPR-12281