fix: overwriting already existing query's context in childQueryOf

This is a fix for an edge case, where a query context could be already set in a `Model.query()` call or in a custom QueryBuilder's constructor.
In our case we are defining columns with a custom `@DefaultOrderBy(columns)` decorator, they are stored to query's context after query builder is instantiated, and eventually are read in `query.onBuild()` hook to apply sorting for a query, if none other was provided to it in the meantime.
But when coupled with fetching graphs, where both parent and children models have `@DefaultOrderBy` configured, child model's query context (eager query to fetch models for a relation) was being completely overwritten with parent's query context, and that, in our case, resulted in using invalid columns to be passed to `orderBy()`, resulting in a sql error.

lib/queryBuilder/QueryBuilderOperationSupport.js

@@ -153,6 +153,7 @@ class QueryBuilderOperationSupport {
 
   childQueryOf(query, { fork, isInternalQuery } = {}) {
     if (query) {
+      let currentCtx = this.context();
       let ctx = query.internalContext();
 
       if (fork) {
@@ -165,6 +166,7 @@ class QueryBuilderOperationSupport {
 
       this._parentQuery = query;
       this.internalContext(ctx);
+      this.context(currentCtx);
 
       // Use the parent's knex if there was no knex in `ctx`.
       if (this.unsafeKnex() === null) {