patch 8.1.0756: copy() does not make a copy of a Blob

Problem:    copy() does not make a copy of a Blob.
Solution:   Make a copy.
diff --git a/src/testdir/test_blob.vim b/src/testdir/test_blob.vim
index 3eeb3d2..d5f682f 100644
--- a/src/testdir/test_blob.vim
+++ b/src/testdir/test_blob.vim
@@ -112,7 +112,14 @@
 
   call assert_false(b1 is b2)
   let b2 = b1
+  call assert_true(b1 == b2)
   call assert_true(b1 is b2)
+  let b2 = copy(b1)
+  call assert_true(b1 == b2)
+  call assert_false(b1 is b2)
+  let b2 = b1[:]
+  call assert_true(b1 == b2)
+  call assert_false(b1 is b2)
 
   call assert_fails('let x = b1 > b2')
   call assert_fails('let x = b1 < b2')