Vorto repository REST fails on dictionary as a return type

[e-grigorov]

The issue can be reproduced with a model like:

namespace test
version 1.0.0
displayname "ReturnDictionary"
description "Functionblock for ReturnDictionary"

functionblock ReturnDictionary {
	operations {
	  mandatory getDictionary() returns dictionary "Returns the property attributes value."
	}
}

The request URI:
http://localhost:8080/infomodelrepository/api/v1/models/test%3AReturnDictionary%3A1.0.0/content

The fail:

java.lang.ClassCastException: org.eclipse.vorto.core.api.model.functionblock.impl.ReturnDictonaryTypeImpl cannot be cast to org.eclipse.vorto.core.api.model.functionblock.ReturnObjectType
        at org.eclipse.vorto.repository.web.core.ModelDtoFactory.createOperation(ModelDtoFactory.java:260) ~[repository-core-0.10.0-SNAPSHOT.jar!/:na]
        at org.eclipse.vorto.repository.web.core.ModelDtoFactory.lambda$createResource$8(ModelDtoFactory.java:214) ~[repository-core-0.10.0-SNAPSHOT.jar!/:na]
        at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) ~[na:1.8.0_102]
        at java.util.Iterator.forEachRemaining(Iterator.java:116) ~[na:1.8.0_102]
        at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801) ~[na:1.8.0_102]
        at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) ~[na:1.8.0_102]
        at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) ~[na:1.8.0_102]
        at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[na:1.8.0_102]
        at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:1.8.0_102]
        at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) ~[na:1.8.0_102]
        at org.eclipse.vorto.repository.web.core.ModelDtoFactory.createResource(ModelDtoFactory.java:214) ~[repository-core-0.10.0-SNAPSHOT.jar!/:na]
        at org.eclipse.vorto.repository.web.core.ModelDtoFactory.createResource(ModelDtoFactory.java:126) ~[repository-core-0.10.0-SNAPSHOT.jar!/:na]
        at org.eclipse.vorto.repository.web.api.v1.ModelController.lambda$getModelContent$0(ModelController.java:109) ~[repository-core-0.10.0-SNAPSHOT.jar!/:na]
        at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374) ~[na:1.8.0_102]
        at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580) ~[na:1.8.0_102]
        at org.eclipse.vorto.repository.web.api.v1.ModelController.getModelContent(ModelController.java:108) ~[repository-core-0.10.0-SNAPSHOT.jar!/:na]
        at org.eclipse.vorto.repository.web.api.v1.ModelController$$FastClassBySpringCGLIB$$cc624ad9.invoke(<generated>) ~[repository-core-0.10.0-SNAPSHOT.jar!/:na]
        at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204) ~[spring-core-4.3.17.RELEASE.jar!/:4.3.17.RELEASE]
        at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:736) ~[spring-aop-4.3.17.RELEASE.jar!/:4.3.17.RELEASE]
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157) ~[spring-aop-4.3.17.RELEASE.jar!/:4.3.17.RELEASE]
        at org.springframework.security.access.intercept.aopalliance.MethodSecurityInterceptor.invoke(MethodSecurityInterceptor.java:69) ~[spring-security-core-4.2.6.RELEASE.jar!/:4.2.6.RELEASE]
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