import models fix for csharp (#6155)

modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/AbstractCSharpCodegen.java

@@ -579,6 +579,11 @@ public abstract class AbstractCSharpCodegen extends DefaultCodegen implements Co
 
     @Override
     public String toModelName(String name) {
+        // We need to check if import-mapping has a different model for this class, so we use it
+        // instead of the auto-generated one.
+        if (importMapping.containsKey(name)) {
+            return importMapping.get(name);
+        }
         if (!StringUtils.isEmpty(modelNamePrefix)) {
             name = modelNamePrefix + "_" + name;
         }