immutableView
Create a read-only view of a builder. Often, we need a read-only view of a builder that may get modified. This would normally require a round trip invoking HttpRequestBuilder.build and then converting that back to a builder using HttpRequest.toBuilder. Instead, we can create an immutable view of a builder that is cheap to convert to a builder.
Parameters
allowToBuilder
flag controlling how this type will behave when HttpRequest.toBuilder is invoked. When false an exception will be thrown, otherwise it will succeed.