moved scheduler initialization to constructor

Created a second builder method which allows to configure the scheduler as a parameter. The default builder method now uses the non deprecated <code>Schedulers.boundedElastic()</code>

Fixes #1711

reactive/src/main/java/feign/reactive/ReactorFeign.java

@@ -25,12 +25,20 @@
 public class ReactorFeign extends ReactiveFeign {
 
   public static Builder builder() {
-    return new Builder();
+    return new Builder(Schedulers.boundedElastic());
+  }
+
+  public static Builder builder(Scheduler scheduler) {
+    return new Builder(scheduler);
   }
 
   public static class Builder extends ReactiveFeign.Builder {
 
-    private Scheduler scheduler = Schedulers.boundedElastic();
+    private final Scheduler scheduler;
+
+    Builder(Scheduler scheduler){
+      this.scheduler = scheduler;
+    }
 
     @Override
     public Feign build() {
@@ -44,10 +52,6 @@ public Builder invocationHandlerFactory(InvocationHandlerFactory invocationHandl
           "Invocation Handler Factory overrides are not supported.");
     }
 
-    public Builder scheduleOn(Scheduler scheduler) {
-      this.scheduler = scheduler;
-      return this;
-    }
   }
 
   private static class ReactorInvocationHandlerFactory implements InvocationHandlerFactory {