Fix RestconfDocumentedExceptionMapper exception

Fix RestconfDocumentedExceptionMapper exception

Seen in CSIT run after jersey 2 upgrade:

2018-05-31T08:47:19,494 | ERROR | qtp315470863-147 | ServerRuntime$Responder          | 190 - org.glassfish.jersey.core.jersey-server - 2.25.1 | An exception has been thrown from an exception mapper class org.opendaylight.netconf.sal.rest.impl.RestconfDocumentedExceptionMapper.
java.lang.UnsupportedOperationException: null
	at java.util.Collections$UnmodifiableList$1.remove(Collections.java:1349) [?:?]
	at jersey.repackaged.com.google.common.collect.TransformedIterator.remove(TransformedIterator.java:53) [185:org.glassfish.jersey.bundles.repackaged.jersey-guava:2.25.1]
	at java.util.AbstractCollection.remove(AbstractCollection.java:293) [?:?]
	at org.opendaylight.netconf.sal.rest.impl.RestconfDocumentedExceptionMapper.toResponse(RestconfDocumentedExceptionMapper.java:104) [346:org.opendaylight.netconf.restconf-nb-bierman02:1.8.0.SNAPSHOT]
	at org.opendaylight.netconf.sal.rest.impl.RestconfDocumentedExceptionMapper.toResponse(RestconfDocumentedExceptionMapper.java:76) [346:org.opendaylight.netconf.restconf-nb-bierman02:1.8.0.SNAPSHOT]

headers.getAcceptableMediaTypes() now returns an immutable list so rework
the code to use stream -> filter - findFirst.

Change-Id: I346c884226c1cfe3afce1b6937176f5397cc3367
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>