Test for null bytes in bson string

Make sure there is no regression in the C driver for:

https://jira.mongodb.org/browse/CDRIVER-2250

src/flags.rs

@@ -36,6 +36,8 @@ pub enum InsertFlag {
     NoValidate
 }
 
+const INSERT_FLAG_NO_VALIDATE: u32 = 1 | 31; // MONGOC_INSERT_NO_VALIDATE defined in macro
+
 impl FlagsValue for Flags<InsertFlag> {
     fn flags(&self) -> u32 {
         if self.flags.is_empty() {
@@ -44,7 +46,7 @@ impl FlagsValue for Flags<InsertFlag> {
             self.flags.iter().fold(0, { |flags, flag|
                 flags | match flag {
                     &InsertFlag::ContinueOnError => bindings::MONGOC_INSERT_CONTINUE_ON_ERROR,
-                    &InsertFlag::NoValidate      => 1 | 31  // MONGOC_INSERT_NO_VALIDATE defined in macro
+                    &InsertFlag::NoValidate      => INSERT_FLAG_NO_VALIDATE
                 }
             })
         }

tests/collection.rs

@@ -72,9 +72,9 @@ fn test_mutation_and_finding() {
 
     let document = doc! {
         "key_1" => "Value 1",
-        "key_2" => "kācaṃ śaknomyattum; nopahinasti mām."
+        "key_2" => "kācaṃ śaknomyattum; nopahinasti mām. \u{0}"
     };
-    assert!(collection.insert(&document, None).is_ok());
+    collection.insert(&document, None).expect("Could not insert document");
     {
         let found_document = collection.find(&document, None).unwrap().next().unwrap().unwrap();
         assert_eq!(
@@ -83,7 +83,7 @@ fn test_mutation_and_finding() {
         );
         assert_eq!(
             found_document.get("key_2").unwrap(),
-            &bson::Bson::String("kācaṃ śaknomyattum; nopahinasti mām.".to_string())
+            &bson::Bson::String("kācaṃ śaknomyattum; nopahinasti mām. \u{0}".to_string())
         );
     }