Closure for creating key has an issue when used with @Transactional annotation

This issue is related to https://github.com/grails-plugins/grails-cache/issues/49.

grails -v
_Grails Version: 3.3.7
Groovy Version: 2.4.15
JVM Version: 1.8.0_222_

The following service method will give the stacktrace below. Issue is with how the key is formed.
It works without the @Transactional annotation. Version of cache plugin is **4.0.3**.

```
@Transactional(readOnly=true)
@Cacheable(value='codes', key={code + '' + layer})
def getCachedData(String code, String layer){
return 'some code'
}
```

Caused by: java.lang.NoSuchMethodError: java.lang.String.plus(Ljava/lang/CharSequence;)Ljava/lang/String;
at helloworld.SimpleService$__tt__getCachedData_closure4.doCall(SimpleService.groovy:89)
at grails.plugin.cache.CustomCacheKeyGenerator.generate(CustomCacheKeyGenerator.groovy:115)
at grails.gorm.transactions.GrailsTransactionTemplate$2.doInTransaction(GrailsTransactionTemplate.groovy:94)
at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:133)

Start with the service reproducer in SimpleService.groovy:89, then inspect CustomCacheKeyGenerator.groovy:115 and GrailsTransactionTemplate.groovy:94 to trace key generation with @Transactional. Reproduce the NoSuchMethodError using Grails 3.3.7 and cache plugin 4.0.3; done means the key is generated and the cached method succeeds with the annotation.

Written by the indexing model from the issue text.