Fix MessagePackReader for multi-segment strings (#1)

* Add failing unit test to show the bug

Author: SteveSandersonMS

src/MessagePack/MessagePackReader.cs

@@ -808,6 +808,7 @@ private string ReadStringSlow(int byteLength)
                     }
                 }
 #endif
+                this.reader.Advance(bytesRead);
             }
 
             string value = new string(charArray, 0, initializedChars);

tests/MessagePack.Tests/MessagePackReaderTests.ReadString.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Buffers;
+using Xunit;
+
+namespace MessagePack.Tests
+{
+    partial class MessagePackReaderTests
+    {
+        [Fact]
+        public void ReadString_HandlesSingleSegment()
+        {
+            var seq = BuildSequence(new[] {
+                (byte)(MessagePackCode.MinFixStr + 2),
+                (byte)'A', (byte)'B' });
+
+            var reader = new MessagePackReader(seq);
+            var result = reader.ReadString();
+            Assert.Equal("AB", result);
+        }
+
+        [Fact]
+        public void ReadString_HandlesMultipleSegments()
+        {
+            var seq = BuildSequence(
+                new[] { (byte)(MessagePackCode.MinFixStr + 2), (byte)'A' },
+                new[] { (byte)'B' });
+
+            var reader = new MessagePackReader(seq);
+            var result = reader.ReadString();
+            Assert.Equal("AB", result);
+        }
+
+        ReadOnlySequence<T> BuildSequence<T>(params T[][] segmentContents)
+        {
+            var segments = new TestSequenceSegment<T>[segmentContents.Length];
+            TestSequenceSegment<T> last = null;
+            for (var i = 0; i < segmentContents.Length; i++)
+            {
+                last = segments[i] = new TestSequenceSegment<T>(segmentContents[i], last);
+            }
+
+            return new ReadOnlySequence<T>(segments[0], 0, last, last.Memory.Length);
+        }
+
+        class TestSequenceSegment<T> : ReadOnlySequenceSegment<T>
+        {
+            public TestSequenceSegment(T[] data, TestSequenceSegment<T> prev = null)
+            {
+                Memory = new Memory<T>(data);
+
+                if (prev != null)
+                {
+                    prev.Next = this;
+                    RunningIndex = prev.RunningIndex + prev.Memory.Length;
+                }
+            }
+        }
+    }
+}