Fix refactor to serialize body (#141)

## Changes
In #135 there was
a refactor to ApiClient's methods by introducing `prepareRequest`. This
accidentally removed the serialization of the body. This PR corrects
that issue.

## Tests
Ran failing integration tests, and they passed.

Author: mgyucht

databricks-sdk-java/src/main/java/com/databricks/sdk/core/ApiClient.java

@@ -174,9 +174,23 @@ public <I, O> O DELETE(String path, I in, Class<O> target, Map<String, String> h
     }
   }
 
-  private <I> Request prepareRequest(
-      String method, String path, I in, Map<String, String> headers) {
-    Request req = new Request(method, path);
+  private boolean hasBody(String method) {
+    return !method.equals("GET") && !method.equals("DELETE") && !method.equals("HEAD");
+  }
+
+  private <I> Request prepareBaseRequest(String method, String path, I in)
+      throws JsonProcessingException {
+    if (in == null || !hasBody(method)) {
+      return new Request(method, path);
+    } else {
+      String body = serialize(in);
+      return new Request(method, path, body);
+    }
+  }
+
+  private <I> Request prepareRequest(String method, String path, I in, Map<String, String> headers)
+      throws JsonProcessingException {
+    Request req = prepareBaseRequest(method, path, in);
     setQuery(req, in);
     setHeaders(req, headers);
     return req;
@@ -306,6 +320,9 @@ public <T> T deserialize(String body, JavaType target) throws IOException {
   }
 
   private String serialize(Object body) throws JsonProcessingException {
+    if (body == null) {
+      return null;
+    }
     return mapper.writeValueAsString(body);
   }
 }