Change GenericConversionService to better handle enum

Prior to this commit, given an enum which implements some interface,
GenericConversionService would select the String -> Enum converter even
if a converter for String -> SomeInterface was registered.  This also
affected converters that were registered for String ->
SomeBaseInterface, when SomeInterface extended SomeBaseInterface.

This change modifies the behavior of the private method
getClassHierarchy() by placing Enum.class as late as possible, pretty
much the same way as Object.class is handled.

Issue: SPR-12050
Merge pull request #607 from dharaburda/SPR-12050

* SPR-12050:
  Change GenericConversionService to better handle enum
Add support for inline maps in SpEL expressions

This commit introduces the ability to specify an inline map in
an expression. The syntax is similar to inline lists and of
the form: "{key:value,key2:value}". The keys can optionally
be quoted. The documentation is also updated with information
on the syntax.

Issue: SPR-9472
Update references to RFC 2616

Replace references to the old RFC 2616 (HTTP 1.1) with references
to the new RFCs 7230 to 7235.

This commit also deprecates:
 - HttpStatus.USE_PROXY
 - HttpStatus.REQUEST_ENTITY_TOO_LARGE in favor of HttpStatus.PAYLOAD_TOO_LARGE
 - HttpStatus.REQUEST_URI_TOO_LONG in favor of HttpStatus.URI_TOO_LONG

Issue: SPR-12067