Revert to creating index asynchronously

Creating the index synchronously was required to get an up to version of
its settings, in order to compare them to the user configuration and
decide whether or not to update settings. Unfortunately this made
practically every operation have a synchronous component causing a lot
of problems down the line.
This commit reverts to using create_index asynchronously to avoid said
issues, and represents the settings of a nonexistent index as an empty
hash, which will queue an update_settings if the user has specified any
settings at all.

lib/meilisearch-rails.rb

@@ -261,7 +261,7 @@ def initialize(index_uid, raise_on_failure, options)
         @raise_on_failure = raise_on_failure.nil? || raise_on_failure
 
         SafeIndex.log_or_throw(nil, @raise_on_failure) do
-          client.create_index!(index_uid, { primary_key: primary_key })
+          client.create_index(index_uid, { primary_key: primary_key })
         end
 
         @index = client.index(index_uid)
@@ -306,7 +306,7 @@ def settings(*args)
         SafeIndex.log_or_throw(:settings, @raise_on_failure) do
           @index.settings(*args)
         rescue ::MeiliSearch::ApiError => e
-          return {} if e.code == 404 # not fatal
+          return {} if e.code == 'index_not_found' # not fatal
 
           raise e
         end

spec/integration_spec.rb

@@ -1136,23 +1136,22 @@
     let(:index_instance) { instance_double(MeiliSearch::Index, settings: nil, update_settings: nil) }
     let(:slow_client) { instance_double(MeiliSearch::Client, index: index_instance) }
 
+    before do
+      allow(slow_client).to receive(:create_index)
+      allow(MeiliSearch::Rails).to receive(:client).and_return(slow_client)
+    end
+
     it 'does not raise error timeouts on reindex' do
       allow(index_instance).to receive(:add_documents).and_raise(MeiliSearch::TimeoutError)
-      allow(slow_client).to receive(:create_index!).and_return(index_instance)
-
-      allow(MeiliSearch::Rails).to receive(:client).and_return(slow_client)
 
       expect do
         Vegetable.create(name: 'potato')
       end.not_to raise_error
     end
 
     it 'does not raise error timeouts on data addition' do
-      allow(slow_client).to receive(:create_index!).and_raise(MeiliSearch::TimeoutError)
       allow(index_instance).to receive(:add_documents).and_return(nil)
 
-      allow(MeiliSearch::Rails).to receive(:client).and_return(slow_client)
-
       expect do
         Vegetable.ms_reindex!
       end.not_to raise_error