fix ContextBuilder.copyFrom NPE (#15)

base `copyOnWrite*Attributes` on respective objects nullability
otherwise they will not be initialized causing `NPE`

Author: tanderson-ld

src/main/java/com/launchdarkly/sdk/ContextBuilder.java

@@ -380,8 +380,8 @@ ContextBuilder copyFrom(LDContext context) {
     anonymous = context.isAnonymous();
     attributes = context.attributes;
     privateAttributes = context.privateAttributes;
-    copyOnWriteAttributes = true;
-    copyOnWritePrivateAttributes = true;
+    copyOnWriteAttributes = context.attributes != null;
+    copyOnWritePrivateAttributes = context.privateAttributes != null;
     return this;
   }
 

src/test/java/com/launchdarkly/sdk/ContextBuilderTest.java

@@ -10,6 +10,7 @@
 import static org.hamcrest.Matchers.emptyIterable;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.notNullValue;
 import static org.hamcrest.Matchers.nullValue;
 
 @SuppressWarnings("javadoc")
@@ -131,4 +132,14 @@ public void builderFromContext() {
       }
     }
   }
+
+  @Test
+  public void doesNotThrowNPEWhenReusingContext() {
+    LDContext initialContext = LDContext.builder("123456").build();
+    LDContext downstreamContext = LDContext.builderFromContext(initialContext)
+            .set("some_attribute", "someValue")
+            .build();
+
+    assertThat(downstreamContext, notNullValue());
+  }
 }