CSHARP-3876: Builders<T>.Filter.Regex(s => s.Name, keyword) doesn't work. (#628)

CSHARP-3876: Builders<T>.Filter.Regex(s => s.Name, keyword) Doesn't Work.

Author: DmitryLukyanov

src/MongoDB.Bson/IO/CStringUtf8Encoding.cs

@@ -56,7 +56,7 @@ public static int GetBytes(string value, byte[] bytes, int byteIndex, UTF8Encodi
                 else
                 {
                     // let fallback encoding handle surrogate pairs
-                    var bytesWritten = fallbackEncoding.GetBytes(value, 0, value.Length, bytes, byteIndex);
+                    var bytesWritten = fallbackEncoding.GetBytes(value, 0, value.Length, bytes, initialByteIndex);
                     if (Array.IndexOf<byte>(bytes, 0, initialByteIndex, bytesWritten) != -1)
                     {
                         throw new ArgumentException("A CString cannot contain null bytes.", "value");

tests/MongoDB.Bson.Tests/IO/CStringUtf8EncodingTests.cs

@@ -53,6 +53,7 @@ public void GetMaxByteCount_should_return_expected_result(
         [InlineData("\\uffff", 0, 3, new byte[] { 0xef, 0xbf, 0xbf })]
         [InlineData("\\uffff", 1, 3, new byte[] { 0xef, 0xbf, 0xbf })]
         [InlineData("\\ud800\\udc00", 0, 4, new byte[] { 0xf0, 0x90, 0x80, 0x80 })] // surrogate pair
+        [InlineData("a😋😋a", 0, 10, new byte[] { 0x61, 0xf0, 0x9F, 0x98, 0x8B, 0xf0, 0x9F, 0x98, 0x8B, 0x61 })] // surrogate pair
         public void GetBytes_should_return_expected_result(
             string value,
             int byteIndex,