Added toString at jst_engine data evaluate

When you try to compile some assets with ```*.eco```, occurs the error that replace method cannot be found for ```this.data``` object at jst_engine ```evaluate``` function.
It happens because at line 61 the ```this.data``` object isn't a string yet. So I attached ```toString()``` to it and the error stopped to happen, compiles with success all files, including ```.eco``` files.
It's only a change at line 61 to compile ```.eco``` files with success.