Fix several protocol test warnings

Author: kstich

smithy-aws-protocol-tests/model/restJson1/http-content-type.smithy

@@ -283,7 +283,6 @@ apply TestGetNoPayload @httpRequestTests([
 /// These tests are to ensure we do not attach a body or related headers
 /// (Content-Type) to a POST operation with no modeled payload.
 ///
-@readonly
 @http(uri: "/no_payload", method: "POST")
 operation TestPostNoPayload {
     input: TestNoPayloadInputOutput,
@@ -363,7 +362,6 @@ apply TestGetNoInputNoPayload @httpRequestTests([
 /// These tests are to ensure we do not attach a body or related headers
 /// (Content-Type) to a POST operation with no modeled input.
 ///
-@readonly
 @http(uri: "/no_input_no_payload", method: "POST")
 operation TestPostNoInputNoPayload {
     output: TestNoPayloadInputOutput

smithy-aws-protocol-tests/model/restJson1/http-string-payload.smithy

@@ -71,6 +71,7 @@ enum StringEnum {
         code: 200
     }
 ])
+@suppress(["UnstableTrait"])
 @httpMalformedRequestTests([
     {
         id: "RestJsonStringPayloadNoContentType",

smithy-aws-protocol-tests/model/restXml/content-type.smithy

@@ -10,6 +10,7 @@ use smithy.test#httpRequestTests
 
 /// The example tests how servers must support requests
 /// containing a `Content-Type` header with parameters.
+@idempotent
 @http(uri: "/ContentTypeParameters", method: "PUT")
 operation ContentTypeParameters {
     input: ContentTypeParametersInput,