Correct behavior for NSMutableData's replaceBytes(in:withBytes:length:) method

Foundation/NSData.swift

@@ -944,12 +944,25 @@ open class NSMutableData : NSData {
         }
 
     }
-
+
+    /// Replaces with a given set of bytes a given range within the contents of the receiver.
+    ///
+    /// If the length of range is not equal to replacementLength, the receiver is resized to 
+    /// accommodate the new bytes. Any bytes past range in the receiver are shifted to accommodate
+    /// the new bytes. You can therefore pass NULL for replacementBytes and 0 for replacementLength
+    /// to delete bytes in the receiver in the range range. You can also replace a range (which
+    /// might be zero-length) with more bytes than the length of the range, which has the effect of
+    /// insertion (or “replace some and insert more”).
+    ///
+    /// - Parameter range: range within the receiver's contents to replace with bytes. The range must not exceed the bounds of the receiver.
+    /// - Parameter replacementBytes: data to insert into the receiver's contents.
+    /// - Parameter replacementLength:  number of bytes to take from `replacementBytes`.
+    ///
+    /// - Note: `replacementLength` must be less than or equal to the
+    ///   size of the buffer pointed to by `replacementBytes`. 
     open func replaceBytes(in range: NSRange, withBytes replacementBytes: UnsafeRawPointer?, length replacementLength: Int) {
-        if let replacementBytes = replacementBytes {
-            let bytePtr = replacementBytes.bindMemory(to: UInt8.self, capacity: replacementLength)
-            CFDataReplaceBytes(_cfMutableObject, CFRangeMake(range.location, range.length), bytePtr, replacementLength)
-        }
+        let bytePtr = replacementBytes?.bindMemory(to: UInt8.self, capacity: replacementLength)
+        CFDataReplaceBytes(_cfMutableObject, CFRangeMake(range.location, range.length), bytePtr, replacementLength)
     }
 }
 

TestFoundation/TestNSData.swift

@@ -421,8 +421,14 @@ class TestNSData: XCTestCase {
         let replacement = makeData([8, 9, 10])
         mData.replaceBytes(in: NSMakeRange(1, 3), withBytes: replacement.bytes,
             length: 3)
-        let expected = makeData([0, 8, 9, 10, 0])
+        var expected = makeData([0, 8, 9, 10, 0])
         XCTAssertEqual(mData, expected)
+
+        // test removing bytes with nil replacementBytes parameter
+        mData.replaceBytes(in: NSMakeRange(1, 3), withBytes: nil, length: 0)
+        expected = makeData([0, 0])
+        XCTAssertEqual(mData, expected)
+
     }
 
     func test_initDataWithCapacity() {