Make AbstractCapabilities implement toJson

The capabilities subclasses were the only thing using `asMap`
for serialisation to JSON. Everywhere else, we use `toJson`.
Might as well make it consistent.

Author: shs96c

java/client/src/org/openqa/selenium/AbstractCapabilities.java

@@ -102,6 +102,10 @@ public Map<String, Object> asMap() {
     return Collections.unmodifiableMap(caps);
   }
 
+  public Map<String, Object> toJson() {
+    return asMap();
+  }
+
   /**
    * Subclasses can use this to add information that isn't always in the capabilities map.
    */