FileManager: Implement subpaths(atPath path: String) -> [String]?

Foundation/FileManager.swift

@@ -951,7 +951,7 @@ open class FileManager : NSObject {
     /* subpathsAtPath: returns an NSArray of all contents and subpaths recursively from the provided path. This may be very expensive to compute for deep filesystem hierarchies, and should probably be avoided.
      */
     open func subpaths(atPath path: String) -> [String]? {
-        NSUnimplemented()
+        return try? subpathsOfDirectory(atPath: path)
     }
 
     /* These methods are provided here for compatibility. The corresponding methods on NSData which return NSErrors should be regarded as the primary method of creating a file from an NSData or retrieving the contents of a file as an NSData.

TestFoundation/TestFileManager.swift

@@ -584,13 +584,16 @@ class TestFileManager : XCTestCase {
             XCTAssertTrue(entries.contains("item2"))
             XCTAssertTrue(entries.contains("sub"))
             XCTAssertTrue(entries.contains("sub/item3"))
+            XCTAssertEqual(fm.subpaths(atPath: path), entries)
         }
         catch _ {
             XCTFail()
         }
 
         do {
             // Check a bad path fails
+            XCTAssertNil(fm.subpaths(atPath: "/..."))
+
             let _ = try fm.subpathsOfDirectory(atPath: "/...")
             XCTFail()
         }