(PUP-10928) Fix indirected_routes_spec.rb for puppet#main

Unlike puppet#7.x (where the :allow_pson_serialization setting was first
added), by default in puppet#main, pson is not a supported format so the rspec
tests for :allow_pson_serialization were not able to test the desired behavior.
This commit stubs out Puppet.features.pson? in those tests to stimulate that
the puppet-pson library is present so pson is a supported format returned by
accepted_response_formatters.

Author: AriaXLi

spec/unit/network/http/api/indirected_routes_spec.rb

@@ -216,6 +216,7 @@
       data = Puppet::IndirectorTesting.new("my data")
       indirection.save(data, "my data")
       request = a_request_that_finds(data, :accept_header => "unknown, text/pson")
+      allow(Puppet.features).to receive(:pson?).and_return(true)
       allow(data).to receive(:to_pson).and_raise(Puppet::Network::FormatHandler::FormatError, 'Could not render to Puppet::Network::Format[pson]: source sequence is illegal/malformed utf-8')
 
       expect {
@@ -267,6 +268,7 @@
       data = Puppet::IndirectorTesting.new("my data")
       indirection.save(data, "my data")
       request = a_request_that_searches(Puppet::IndirectorTesting.new("my"), :accept_header => "unknown, text/pson")
+      allow(Puppet.features).to receive(:pson?).and_return(true)
       allow(data).to receive(:to_pson).and_raise(Puppet::Network::FormatHandler::FormatError, 'Could not render to Puppet::Network::Format[pson]: source sequence is illegal/malformed utf-8')
 
       expect {