Write config - default to the connection string writeconcern is available. (#134)

SPARK-392

Author: rozza

src/main/java/com/mongodb/spark/sql/connector/config/WriteConfig.java

@@ -344,7 +344,10 @@ private String getNamespaceDescription(final CollectionsConfig collectionsConfig
   }
 
   private WriteConcern createWriteConcern() {
-    WriteConcern writeConcern = WriteConcern.ACKNOWLEDGED;
+    WriteConcern writeConcern = getConnectionString().getWriteConcern();
+    if (writeConcern == null) {
+      writeConcern = WriteConcern.ACKNOWLEDGED;
+    }
     try {
       if (containsKey(WRITE_CONCERN_W_CONFIG)) {
         try {

src/test/java/com/mongodb/spark/sql/connector/config/MongoConfigTest.java

@@ -126,6 +126,18 @@ void testMongoConfigWriteConcern() {
     writeConfig = writeConfig.withOption(
         WriteConfig.WRITE_PREFIX + WriteConfig.WRITE_CONCERN_W_CONFIG, "region1");
     assertEquals(new WriteConcern("region1"), writeConfig.getWriteConcern());
+
+    writeConfig = MongoConfig.createConfig(CONFIG_MAP).toWriteConfig();
+    writeConfig.withOption(MongoConfig.CONNECTION_STRING_CONFIG, "mongodb://localhost/");
+    assertEquals(WriteConcern.ACKNOWLEDGED, writeConfig.getWriteConcern());
+
+    writeConfig = writeConfig.withOption(
+        MongoConfig.CONNECTION_STRING_CONFIG, "mongodb://localhost/?w=majority");
+    assertEquals(WriteConcern.MAJORITY, writeConfig.getWriteConcern());
+
+    // Explicit configuration beats connection string
+    writeConfig = writeConfig.withOption(WriteConfig.WRITE_CONCERN_W_CONFIG, "3");
+    assertEquals(3, writeConfig.getWriteConcern().getW());
   }
 
   @Test