Add minor tests

Author: malczuuu

src/test/java/io/github/malczuuu/problem4j/core/ProblemTests.java

@@ -0,0 +1,52 @@
+package io.github.malczuuu.problem4j.core;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotSame;
+
+import java.net.URI;
+import java.util.Arrays;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+import org.junit.jupiter.api.Test;
+
+class ProblemTests {
+
+  @Test
+  void givenProblemStatus_shouldSetTitleAndStatus() {
+    Problem problem = Problem.builder().status(ProblemStatus.MULTI_STATUS).build();
+
+    assertEquals(ProblemStatus.MULTI_STATUS.getTitle(), problem.getTitle());
+    assertEquals(ProblemStatus.MULTI_STATUS.getStatus(), problem.getStatus());
+  }
+
+  @Test
+  void givenProblem_whenToBuilder_shouldBeAbleToRecreateOriginal() {
+    Problem problem =
+        Problem.builder()
+            .type(URI.create("http://example.org/problem1"))
+            .title("Problem1")
+            .status(400)
+            .detail("this is problem1")
+            .instance(URI.create("http://example.org/instance1"))
+            .extension("extCode1", "extValue1")
+            .extension(Problem.extension("extCode2", "extValue2"))
+            .extension(
+                Arrays.asList(
+                    Problem.extension("extCode3", "extValue3"),
+                    Problem.extension("extCode4", "extValue4")))
+            .extension(
+                Stream.of(
+                        Problem.extension("extCode5", "extValue5"),
+                        Problem.extension("extCode6", "extValue6"),
+                        Problem.extension("extCode7", "extValue7"))
+                    .collect(Collectors.toMap(Problem.Extension::getKey, Function.identity())))
+            .build();
+
+    ProblemBuilder builder = problem.toBuilder();
+    Problem copy = builder.build();
+
+    assertNotSame(problem, copy);
+    assertEquals(problem, copy);
+  }
+}