Fix ruby httpx test (#16765)

* fix setup of body params (may be nil)

* fix httpx adapter issues

treating DNS resolution errors as ConnectionFailed; removing multipart header set by the openapi-generated code, as it does not contain boundary, and interferes with the generation from httpx, which appropriately deals with mime-types already

samples/client/petstore/ruby-httpx/spec/custom/pet_spec.rb

@@ -106,7 +106,7 @@ describe "Pet" do
     it "should fetch a pet object with http info" do
       pet, status_code, headers = @pet_api.get_pet_by_id_with_http_info(@pet_id)
       expect(status_code).to eq(200)
-      expect(headers['Content-Type']).to eq('application/json')
+      expect(headers['content-type']).to eq('application/json')
       expect(pet).to be_a(Petstore::Pet)
       expect(pet.id).to eq(@pet_id)
       expect(pet.name).to eq("RUBY UNIT TESTING")
@@ -123,8 +123,8 @@ describe "Pet" do
         # skip the check as the response contains a timestamp that changes on every response
         # expect(e.message).to eq("Error message: the server returns an error\nHTTP status code: 404\nResponse headers: {\"Date\"=>\"Tue, 26 Feb 2019 04:35:40 GMT\", \"Access-Control-Allow-Origin\"=>\"*\", \"Access-Control-Allow-Methods\"=>\"GET, POST, DELETE, PUT\", \"Access-Control-Allow-Headers\"=>\"Content-Type, api_key, Authorization\", \"Content-Type\"=>\"application/json\", \"Connection\"=>\"close\", \"Server\"=>\"Jetty(9.2.9.v20150224)\"}\nResponse body: {\"code\":1,\"type\":\"error\",\"message\":\"Pet not found\"}")
         expect(e.response_body).to eq('{"code":1,"type":"error","message":"Pet not found"}')
-        expect(e.response_headers).to include('Content-Type')
-        expect(e.response_headers['Content-Type']).to eq('application/json')
+        expect(e.response_headers).to include('content-type')
+        expect(e.response_headers['content-type']).to eq('application/json')
       end
     end