sets array size to max size allowed for dont_split true

If a bigger array is created with this `typesize` the bit `dont_split` would be
set to false.

test/test_headers.py

@@ -156,14 +156,14 @@ def test_decode_blosc_header_deactivate_shuffle():
 
 
 def test_decode_blosc_header_uncompressible_data():
-    array_ = np.asarray(np.random.randn(23),
+    array_ = np.asarray(np.random.randn(255),
                         dtype=np.float32).tostring()
     blosc_args = BloscArgs()
     blosc_args.shuffle = True
     compressed = blosc.compress(array_, **blosc_args)
     header = decode_blosc_header(compressed)
     expected = {'versionlz': 1,
-                'blocksize': 88,
+                'blocksize': 1016,
                 'ctbytes': len(array_) + 16,  # original + 16 header bytes
                 'version': 2,
                 'flags': 0x13,  # 1 for shuffle 2 for non-compressed 4 for small blocksize