fix: cube validation from updating BasePreAggregation (#285). Thanks to @ferrants!

Author: ferrants

packages/cubejs-schema-compiler/compiler/CubeValidator.js

@@ -148,20 +148,20 @@ const cubeSchema = Joi.object().keys({
     description: Joi.string()
   })),
   preAggregations: Joi.object().pattern(identifierRegex, Joi.alternatives().try(
-    Joi.object().keys(Object.assign(BasePreAggregation, {
+    Joi.object().keys(Object.assign({}, BasePreAggregation, {
       type: Joi.any().valid('autoRollup').required(),
       maxPreAggregations: Joi.number()
     })),
-    Joi.object().keys(Object.assign(BasePreAggregation, {
+    Joi.object().keys(Object.assign({}, BasePreAggregation, {
       type: Joi.any().valid('originalSql').required()
     })),
-    Joi.object().keys(Object.assign(BasePreAggregation, {
+    Joi.object().keys(Object.assign({}, BasePreAggregation, {
       type: Joi.any().valid('rollup').required(),
       measureReferences: Joi.func(),
       dimensionReferences: Joi.func(),
       segmentReferences: Joi.func()
     })),
-    Joi.object().keys(Object.assign(BasePreAggregation, {
+    Joi.object().keys(Object.assign({}, BasePreAggregation, {
       type: Joi.any().valid('rollup').required(),
       measureReferences: Joi.func(),
       dimensionReferences: Joi.func(),