feat: add diff method PullRequestClient (#149)

* Add test method for PullRequestClient

* fix test

Author: mkarimi23

src/main/java/com/spotify/github/v3/clients/PullRequestClient.java

@@ -248,6 +248,22 @@ public CompletableFuture<Reader> patch(final int number) {
         });
   }
 
+  public CompletableFuture<Reader> diff(final int number) {
+    final String path = String.format(PR_NUMBER_TEMPLATE, owner, repo, number);
+    final Map<String, String> extraHeaders = ImmutableMap.of(
+        HttpHeaders.ACCEPT, "application/vnd.github.diff"
+    );
+    log.debug("Fetching pull diff from " + path);
+    return github.request(path, extraHeaders)
+        .thenApply(response -> {
+          final var body = response.body();
+          if (isNull(body)) {
+            return Reader.nullReader();
+          }
+          return body.charStream();
+        });
+  }
+
   private CompletableFuture<List<PullRequestItem>> list(final String parameterPath) {
     final String path = String.format(PR_TEMPLATE + parameterPath, owner, repo);
     log.debug("Fetching pull requests from " + path);

src/test/java/com/spotify/github/v3/clients/PullRequestClientTest.java

@@ -294,4 +294,37 @@ public void testGetPatch() throws Throwable {
 
     assertEquals(getFixture("patch.txt"), IOUtils.toString(patchReader));
   }
+
+  @Test
+  public void testGetDiff() throws Throwable {
+    final Call call = mock(Call.class);
+    final ArgumentCaptor<Callback> capture = ArgumentCaptor.forClass(Callback.class);
+    doNothing().when(call).enqueue(capture.capture());
+
+    final Response response =
+        new Response.Builder()
+            .code(200)
+            .protocol(Protocol.HTTP_1_1)
+            .message("OK")
+            .body(
+                ResponseBody.create(
+                    MediaType.get("application/vnd.github.diff"),
+                    getFixture("diff.txt")))
+            .request(new Request.Builder().url("http://localhost/").build())
+            .build();
+
+    when(client.newCall(any())).thenReturn(call);
+
+    final PullRequestClient pullRequestClient =
+        PullRequestClient.create(github, "owner", "repo");
+
+    final CompletableFuture<Reader> result =
+        pullRequestClient.diff(1);
+
+    capture.getValue().onResponse(call, response);
+
+    Reader diffReader = result.get();
+
+    assertEquals(getFixture("diff.txt"), IOUtils.toString(diffReader));
+  }
 }

src/test/resources/com/spotify/github/v3/clients/diff.txt

@@ -0,0 +1,9 @@
+diff --git a/filename b/filename
+index 01a9f34..500bb03 100644
+--- a/nf
++++ b/nf
+@@ -1,3 +1,4 @@
+ asdf
++asdf
+--
+2.39.0