Schedules the request to be executed at some point in the future.
<p>The {@link OkHttpClient#dispatcher dispatcher} defines when the request will run: usually
immediately unless there are several other requests currently being executed.
<p>This client will later call back {@code responseCallback} with either an HTTP response or a
failure exception.
@throws IllegalStateException when the call has already been executed.
Schedules the request to be executed at some point in the future.
<p>The {@link OkHttpClient#dispatcher dispatcher} defines when the request will run: usually immediately unless there are several other requests currently being executed.
<p>This client will later call back {@code responseCallback} with either an HTTP response or a failure exception.
@throws IllegalStateException when the call has already been executed.