ServerEndpointExporter can initialize itself based on a late-provided ServletContext as well (for Boot)

Also allows for direct setting of a ServerContainer and for custom triggering of endpoint registration.

Issue: SPR-12109
Polishing
@PropertySource gets parsed as early as possible

Other PropertySources and in particular @ComponentScan can benefit from previously declared property sources on the same configuration class.

Issue: SPR-12110
Issue: SPR-12111
PropertySource implementations perform conversion to String arrays via StringUtils (getting rid of EMPTY_NAMES_ARRAY)
Rework @PropertySource early parsing logic

Rework the @PropertySource parsing logic recently changed in commit
7c608886 to deal with the same source appearing on a @Configuration
class and an @Import class.

Processing now occurs in a single sweep, with any previously added
sources being converted to a CompositePropertySource.

Issue: SPR-12115
Fix failing CompositePropertySourceTests

Fix CompositePropertySourceTests which fails when run we debug logging.
ResponseEntity provides static "badRequest()" convenience method as well

Issue: SPR-12112
Revised ResourcePropertySource in order to avoid ConfigurationClassProcessor's AnnotationPropertySource subclass

Issue: SPR-12115
Polishing

Issue: SPR-12112
DispatcherServlet's checkMultipart detects wrapped MultipartRequest as well

Issue: SPR-12114
MessageBuilder avoids intermediate HashMap for MessageHeaders building

Issue: SPR-11468
Polishing
Polishing
Polishing