[BUG][JAVA][spring-mvc] Generated Code for Map of Maps

Created by: emileonhardt

Fix for https://github.com/OpenAPITools/openapi-generator/issues/4840 The Java-Code (spring-mvc) generated from OpenAPI-YAML for a Map of Maps ist not syntactically correct.

This is caused by SpringCodegen.java not handling Maps of Maps return types correctly. Its a simple mistake in applying String.split() to extract the Key-Type of the outer Map.

The following Unit Test shows the problem.

@Test
   public void doDataTypeAssignmentNestedMapReturnType() {
        final SpringCodegen codegen = new SpringCodegen();
        codegen.doDataTypeAssignment("Map<String, Map<String, String>>", new SpringCodegen.DataTypeAssigner() {
            @Override
            public void setReturnType(String returnType) {
               assertEquals(returnType,"Map<String, String>");
            }

            @Override
            public void setReturnContainer(String returnContainer) {
                assertEquals(returnContainer, "Map");
            }
        });
    }

Unfortunately, the visibility of doDataTypeAssignment() and DataTypeAssigner is private, so I can't check in the Test.

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