fix: prevent unchecked warnings in gax-httpjson (#1306)

Author: miraleung

gax-httpjson/src/main/java/com/google/api/gax/httpjson/ApiMessageHttpRequestFormatter.java

@@ -75,6 +75,7 @@ ApiMessageHttpRequestFormatter.Builder<RequestT> newBuilder() {
  return new ApiMessageHttpRequestFormatter.Builder<>();
  }
 
+ @SuppressWarnings("unchecked")
  @Override
  public Map<String, List<String>> getQueryParamNames(RequestT apiMessage) {
  Set<String> paramNames = getQueryParamNames();

gax-httpjson/src/main/java/com/google/api/gax/httpjson/ApiMethodDescriptor.java

@@ -50,7 +50,7 @@ public abstract class ApiMethodDescriptor<RequestT, ResponseT> {
  public abstract String getHttpMethod();
 
  public static <RequestT, ResponseT> Builder<RequestT, ResponseT> newBuilder() {
- return new AutoValue_ApiMethodDescriptor.Builder();
+ return new AutoValue_ApiMethodDescriptor.Builder<RequestT, ResponseT>();
  }
 
  @AutoValue.Builder