Fix media type lookup case sensitivity

[mhartsock]

Fixed a bug where the URL content negotiation "format" parameter values
were case sensitive and only lowercase values were accepted. For
example, URL query parameter format=json returned the appropriate JSON
response but format=JSON resulted in a
HttpMediaTypeNotAcceptableException and returned:

406 - The resource identified by this request is only capable of
generating responses with characteristics not acceptable according to
the request "accept" headers.

When the MappingMediaTypeFileExtensionResolver is constructed, it is
passed a map containing the media type key to MediaType mappings
defined in the ContentNegotiationConfigurer. In the constructor of
MappingMediaTypeFileExtensionResolver, the keys are converted to
lowercase and the mappings of keys to MediaTypes are added to the
ConcurrentMap<String, MediaType> mediaTypes using the lowercase
version of the keys. However, when retrieving the MediaType from a key
in the lookupMediaType method, no conversion to lowercase is performed
so any value for the URL "format" parameter other than the lowercase
version will not return the proper MediaType result.

On May 1st, 2014, a change was made to
ParameterContentNegotiationStrategy to handle cases where the content
negotiation format URL parameter does not result in a match for a
MediaType. If no match is found, a HttpMediaTypeNotAcceptableException
is thrown resulting in the 406 response above. Prior to this commit, a
null was returned instead of throwing an exception so this issue was
hidden and appeared to function correctly.

To make the media type lookup case insensitive, added a line to the
lookupMediaType method in MediaTypeFileExtensionResolver to first
convert the extension (media type key) to lowercase prior to attempting
to retrieve it from the mediaTypes map.

Issue: SPR-13747

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[rstoyanchev]

This is now merged. Thanks for the thorough report and pull request!