Fix unwanted side effects of boost, and cascading of combineWith

The `boost` search option was mistakenly causing boosted fields to be
included in search, even when they were not in the `fields` search
option.

The `combineWith` search option was not properly taking its default from
the `SearchOptions` argument when using `search` with a `QuerySpec`
tree.

Both issues are fixed by this commit, that also adds corresponding
tests.

src/MiniSearch.test.js

@@ -1003,12 +1003,18 @@ describe('MiniSearch', () => {
       })
     })
 
-    it('searches in the given fields', () => {
+    it('searches only selected fields', () => {
       const results = ms.search('vita', { fields: ['title'] })
       expect(results).toHaveLength(1)
       expect(results[0].id).toEqual(3)
     })
 
+    it('searches only selected fields even if other fields are boosted', () => {
+      const results = ms.search('vita', { fields: ['title'], boost: { text: 2 } })
+      expect(results).toHaveLength(1)
+      expect(results[0].id).toEqual(3)
+    })
+
     it('combines results with OR by default', () => {
       const results = ms.search('cammin como sottomarino')
       expect(results.length).toEqual(2)
@@ -1222,6 +1228,48 @@ describe('MiniSearch', () => {
         expect(results.length).toEqual(2)
         expect(results.map(({ id }) => id)).toEqual([3, 2])
       })
+
+      it('uses the search options in the second argument as default', () => {
+        let reference = ms.search({
+          queries: [
+            { fields: ['text'], queries: ['vita'] },
+            { fields: ['title'], queries: ['promessi'] }
+          ]
+        })
+
+        // Boost field
+        let results = ms.search({
+          queries: [
+            { fields: ['text'], queries: ['vita'] },
+            { fields: ['title'], queries: ['promessi'] }
+          ]
+        }, { boost: { title: 2 } })
+
+        expect(results.length).toEqual(reference.length)
+        expect(results.find((r) => r.id === 2).score)
+          .toBeGreaterThan(reference.find((r) => r.id === 2).score)
+
+        // Combine with AND
+        results = ms.search({
+          queries: [
+            { fields: ['text'], queries: ['vita'] },
+            { fields: ['title'], queries: ['promessi'] }
+          ]
+        }, { combineWith: 'AND' })
+
+        expect(results.length).toEqual(0)
+
+        // Combine with AND, then override it with OR
+        results = ms.search({
+          queries: [
+            { fields: ['text'], queries: ['vita'] },
+            { fields: ['title'], queries: ['promessi'] }
+          ],
+          combineWith: 'OR'
+        }, { combineWith: 'AND' })
+
+        expect(results.length).toEqual(reference.length)
+      })
     })
 
     describe('match data', () => {

src/MiniSearch.ts

@@ -1420,7 +1420,7 @@ export default class MiniSearch<T = any> {
     if (typeof query !== 'string') {
       const options = { ...searchOptions, ...query, queries: undefined }
       const results = query.queries.map((subquery) => this.executeQuery(subquery, options))
-      return this.combineResults(results, query.combineWith)
+      return this.combineResults(results, options.combineWith)
     }
 
     const { tokenize, processTerm, searchOptions: globalSearchOptions } = this._options
@@ -1442,7 +1442,7 @@ export default class MiniSearch<T = any> {
     const options: SearchOptionsWithDefaults = { ...this._options.searchOptions, ...searchOptions }
 
     const boosts = (options.fields || this._options.fields).reduce((boosts, field) =>
-      ({ ...boosts, [field]: getOwnProperty(boosts, field) || 1 }), options.boost || {})
+      ({ ...boosts, [field]: getOwnProperty(options.boost, field) || 1 }), {})
 
     const {
       boostDocument,