Allow symbols for credential flow

Because of my production environment, adding a JSON file a little bit of
a pain. However: using the intializer it's readily possible to set my
credentials manually from my environment. I noticed that while all the
credential details can be either a String or a Symbol, the 'flow' (i.e.,
`web`) **must** be a string.

This allows it to accept both a string and a symbol.

spec/google/api_client/client_secrets_spec.rb

@@ -215,7 +215,7 @@ RSpec.describe Google::APIClient::ClientSecrets do
 
       context 'option keys are symbol' do
         let(:symbol_options) do
-          { 'samples' =>
+          { samples:
             {
               access_token:         'sample_access_token',
               auth_uri:             'sample_auth_uri',